CSS Positioned Layout Module Level 3

Editor’s Draft, 27 October 2014

This version:
http://dev.w3.org/csswg/css-position/
Latest version:
http://www.w3.org/TR/css3-positioning/
Previous Versions:
http://www.w3.org/TR/2012/WD-css3-positioning-20120207/
Feedback:
www-style@w3.org with subject line “[css-position] … message topic …” (archives)
Editors:
(Microsoft)
(Microsoft)
Issues list:
in Wiki

Abstract

This module contains the features of CSS level 3 relating to positioning and stacking of elements. It includes and extends the functionality of CSS level 2 ([CSS21]), which builds on CSS level 1 [CSS1]. The main extensions compared to level 2 are the ability to position elements based on CSS Region boxes, the ability to specify different containing blocks for elements and sticky positioning.

Other kinds of layout, such as tables, "floating" boxes, ruby annotations, grid layouts, columns and basic handling of normal "flow" content, are described in other modules. Also, the layout of text inside each line is defined elsewhere.

CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, in speech, etc.

Status of this document

This is a public copy of the editors’ draft. It is provided for discussion only and may change at any moment. Its publication here does not imply endorsement of its contents by W3C. Don’t cite this document other than as work in progress.

The (archived) public mailing list www-style@w3.org (see instructions) is preferred for discussion of this specification. When sending e-mail, please put the text “css-position” in the subject, preferably like this: “[css-position] …summary of comment…

This document was produced by the CSS Working Group (part of the Style Activity).

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This document is governed by the 1 August 2014 W3C Process Document.

Table of Contents

1. Introduction

This section is not normative.

CSS assumes the document layout is modeled as a tree of elements. The unique element that has no parent is called the root element. This module describes how any of the elements from the tree of elements can be arranged independent of document order (i.e. taken out of "flow"). With a positioned element the element may be placed anywhere within the content not specifically respecting the tree of elements order.

In [CSS21], the visual formatting model explained how each element in the document tree generates zero or more boxes according to the box model. This module further explains and extends the positioning scheme. The layout of these boxes is governed by:

The properties defined in this module apply to both continuous media and paged media.

1.1. Module Interactions

This module replaced and extends the positioning scheme features defined in [CSS21] sections:

1.2. Values

This specification follows the CSS property definition conventions from [CSS21].

Value types not defined in this specification are defined in CSS Level 2 Revision 1 [CSS21]. Other CSS modules may expand the definitions of these value types: for example [CSS3VAL], when combined with this module, expands the definition of the <length> value type as used in this specification.

In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the inherit keyword as their property value. For readability it has not been repeated explicitly.

2. The Viewport

User agents for continuous media generally offer users a viewport (a window or other viewing area on the screen) through which users consult a document. User agents may change the document's layout when the viewport is resized (see the initial containing block).

When the viewport is smaller than the area of the canvas on which the document is rendered, the user agent may offer a scrolling mechanism. There is at most one viewport per canvas, but user agents may render to more than one canvas (i.e., provide different views of the same document).

3. Containing Blocks

In CSS, many box positions and sizes are calculated with respect to the edges of a rectangular box called a containing block. In general, generated boxes act as containing blocks for descendant boxes; we say that a box "establishes" the containing block for its descendants. The phrase "a box's containing block" means "the containing block in which the box lives," not the one it generates.

Each box is given a position with respect to its containing block, but it is not confined by this containing block; it may overflow.

3.1. Definition of containing block

The position and size of an element's box(es) are sometimes computed relative to a certain rectangle, called the containing block of the element. The containing block of a static or relative element is defined in the Box Model [CSS3BOX]. The containing block of a sticky element is the same as for a relative element. For fixed, absolute, center and page, it is defined as follows:

  1. If the element has 'positioned: fixed', the containing block is established by the viewport in the case of continuous media or the page area in the case of paged media.
  2. If the element has 'position: absolute', the containing block is established by the nearest ancestor with a position other than static, in the following way:
    1. In the case that the ancestor is block-level, the containing block is formed by the padding edge of the ancestor.
    2. In the case that the ancestor is inline-level, the containing block depends on the direction property of the ancestor:
      1. If the direction is ltr, the top and left of the containing block are the top and left content edges of the first box generated by the ancestor, and the bottom and right are the bottom and right content edges of the last box of the ancestor.
      2. If the direction is rtl, the top and right are the top and right edges of the first box generated by the ancestor, and the bottom and left are the bottom and left content edges of the last box of the ancestor.

        Note, in some cases when a line wraps it may seem as if the left and right positions are swapped.

  3. I the element has 'position: center', the containing block is established by the nearest ancestor with a position other than static, in the following way:
    1. In the case that the ancestor is block-level, the containing block is formed by the padding edge of the ancestor.
    2. In the case that the ancestor is inline-level, the containing block depends on the content edges of the boxes generated by the ancestor. The top most, right most, bottom most and left most content edges of all the boxes generated by the ancestor determine the edges for the containing block.
  4. If the element has 'position: page' and the element is within paged media or within a rerion, the containing block is the initial containing block. This is the viewport or the page area when in paged media. In the case of CSS Regions [CSS3-REGIONS] this is the individual region. When a page positioned element is not in paged media or a region the containing block is determined per the absolute positioning model.
  5. If there is no such ancestor, the containing block is the initial containing block.

In paged media, an absolutely positioned element is positioned relative to its containing block ignoring any page breaks (as if the document were continuous). The element may subsequently be broken over several pages.

For absolutely positioned content that resolves to a position on a page other than the page being laid out (the current page), or resolves to a position on the current page which has already been rendered for printing, printers may place the content:

Note, a block-level element that is split over several pages may have a different width on each page and that there may be device-specific limits.

With no positioning, the containing blocks (C.B.) in the following document:


<!DOCTYPE html>
<html>
    <head>
        <title>Illustration of containing blocks</title>
    </head>
    <body id="body">
        <div id="div1">
        <p id="p1">This is text in the first paragraph...</p>
        <p id="p2">This is text <em id="em1"> in the
        <strong id="strong1">second</strong> paragraph.</em></p>
        </div>
    </body>
</html>

are established as follows:

For box generated by C.B. is established by
htmlinitial C.B. (UA-dependent)
bodyhtml
div1body
p1div1
p2div1
em1p2
strong1p2

If we position "div1":


#div1 { position: absolute; left: 50px; top: 50px }

its containing block is no longer "body"; it becomes the initial containing block (since there are no other positioned ancestor boxes).

If we position "em1" as well:


#div1 { position: absolute; left: 50px; top: 50px }
#em1  { position: absolute; left: 100px; top: 100px }

the table of containing blocks becomes:

For box generated by C.B. is established by
htmlinitial C.B. (UA-dependent)
bodyhtml
div1initial C.B.
p1div1
p2div1
em1div1
strong1em1

By positioning "em1", its containing block becomes the nearest positioned ancestor box (i.e., that generated by "div1").

4. Normal Flow

Boxes in the normal flow belong to a formatting context, which may be block or inline, but not both simultaneously. See the CSS Basic Box Model module [CSS3BOX] for further details about normal flow.

5. Floats

A float is a box that is positioned within content, often left or right on the current line. The most interesting characteristic of a float (or "floated" or "floating" box) is that inline-level content may flow along its side (or be prohibited from doing so by the clear property) or around the floated box. See the CSS 2.1 [CSS21] for details about floating boxes.

6. Positioning schemes

In CSS, a box may be laid out according to three positioning schemes:

  1. Normal flow
    In CSS, normal flow includes block formatting of block-level boxes, inline formatting of inline-level boxes, and relative and sticky positioning of block-level and inline-level boxes.
  2. Floats
    In the float model, a box is first laid out according to the normal flow, then taken out of the flow and positioned, typically to the left or right. Content may flow along the side of a float.
  3. Absolute positioning
    In the absolute positioning model, a box is removed from the normal flow entirely (it has no impact on later siblings) and assigned a position with respect to a containing block.

An element is called out-of-flow if it is floated, absolutely positioned, or is the root element. An element is called in-flow if it is not out-of-flow. The flow of an element A is the set consisting of A and all in-flow elements whose nearest out-of-flow ancestor is A.

6.1. Relative positioning

Once a box has been laid out according to the normal flow or floated, it may be offset relative to this position. This is called relative positioning. Offsetting a box (B1) in this way has no effect on the box (B2) that follows: B2 is given a position as if B1 were not offset and B2 is not re-positioned after B1's offset is applied. This implies that relative positioning may cause boxes to overlap. However, if relative positioning causes an 'overflow: auto' or 'overflow: scroll' box to have overflow, the user agent must allow the user to access this content (at its offset position), which, through the creation of a scrolling mechanism, may affect layout.

A relatively positioned box keeps its normal flow size, including line breaks and the space originally reserved for it.

A relatively positioned box establishes a new a new containing block for absolutely positioned descendants. (This is a common use of relatively positioned boxes.) The section on containing blocks explains when a relatively positioned box establishes a new containing block.

For relatively positioned elements, left and right move the box(es) horizontally, without changing their size. Left moves the boxes to the right, and right moves them to the left. Since boxes are not split or stretched as a result of left or right, the used values are always: left = -right.

If both left and right are auto (their initial values), the used values are 0 (i.e., the boxes stay in their original position).

If left is auto, its used value is minus the value of right (i.e., the boxes move to the left by the value of right).

If right is specified as auto, its used value is minus the value of left.

If neither left nor right is auto, the position is over-constrained, and one of them has to be ignored. If the direction property of the containing block is ltr, the value of left wins and right becomes -left. If direction of the containing block is rtl, right wins and left is ignored.

The following three rules are equivalent:


div.a8 { position: relative; direction: ltr; left: -1em; right: auto }
div.a8 { position: relative; direction: ltr; left: auto; right: 1em }
div.a8 { position: relative; direction: ltr; left: -1em; right: 5em }

The top and bottom properties move relatively positioned element(s) up or down without changing their size. Top moves the boxes down, and bottom moves them up. Since boxes are not split or stretched as a result of top or bottom, the used values are always: top = -bottom.

If top and bottom are auto, their used values are both 0.

If one of them, top or bottom, is auto, the auto value becomes the negative of the other.

If neither top and bottom is auto, bottom is ignored (i.e., the used value of bottom will be minus the value of top).

Note, Although relative positioning could be used as a form of superscripting and subscripting, the line height is not automatically adjusted to take the positioning into consideration. See the description of line height calculations for more information.

Examples of relative positioning are provided in the section comparing normal flow, floats, and absolute positioning.

6.2. Sticky positioning

A stickily positioned box is positioned similarly to a relatively positioned box, but the offset is computed with reference to the nearest ancestor with a scrolling box, or the viewport if no ancestor has a scrolling box.

Specifically, once a box has been laid out according to the normal flow or floated, its sticky offset is computed as described below. Offsetting a box (B1) in this way has no effect on the box (B2) that follows: B2 is given a position as if B1 were not offset and B2 is not re-positioned after B1’s offset is applied. This implies that sticky positioning may cause boxes to overlap. However, if sticky positioning causes an 'overflow: auto' or 'overflow: scroll' box to have overflow, the user agent must allow the user to access this content (at its offset position), which, through the creation of a scrolling mechanism, may affect layout.

A stickily positioned box keeps its normal flow size, including line breaks and the space originally reserved for it.

A stickily positioned box establishes a new a new containing block for absolutely positioned descendants, just as relative positioning does. The section on containing blocks explains when a stickily positioned box establishes a new containing block.

For stickily positioned elements, left, right, top and bottom are offsets from the respective edges of its flow box which are used to constrain the element’s offset. Percentage values of left and right refer to the width of its flow box; percentage values of top and bottom refer to the height of its flow box.

Describe which element font-size-relative units are resolved against

The offset of a stickily positioned box is computed as follows:

  1. A rectangle is computed relative to the containing block of the stickily positioned element, by insetting its flow box rectangle on each side by offsets computed from the left, right, top and bottom properties of the stickily positioned element.

    Say what happens if this rectangle is empty

  2. The intersection is taken between the resulting rectangle, and the containing block of the stickily positioned element. The result, termed the the sticky-constraint rectangle, is a rectangle used to constrain the location of the stickily positioned element.
  3. If the stickily positioned element’s top style is not auto, and the stickily positioned element projects above the top of the sticky-constraint rectangle, the stickily positioned element is moved down until it is fully contained in the sticky-constraint rectangle.
  4. If the stickily positioned element’s bottom style is not auto, and the stickily positioned element projects below the bottom of the sticky-constraint rectangle, the stickily positioned element is moved up until it is fully contained in the sticky-constraint rectangle.
  5. If the stickily positioned element’s left style is not auto, and the stickily positioned element projects outside the left of the sticky-constraint rectangle, the stickily positioned element is moved right until it is fully contained in the sticky-constraint rectangle.
  6. If the stickily positioned element’s right style is not auto, and the stickily positioned element projects outside the right of the sticky-constraint rectangle, the stickily positioned element is moved left until it is fully contained in the sticky-constraint rectangle.

When computing containement of the stickily positioned element within its containing block, margins on the stickily positioned element are taken into account.

Say what happens if it already overflows the containing block

Do marins collapse between the stickily positioned element and its containing block element?

Intersection between the stickily positioned element and the bottom of the sticky-constraint rectangle limits movement in any direction, so the offset never pushes the stickily positioned element outside of its containing block. However, when the element is free to move within its containing block as the page is scrolled, it appears to be pinned to the relevant flow root edges, similarly to a fixed position element.

Note that a stickily positioned element with non-auto top style and auto bottom style will only ever be pushed down by sticky positioning; it will never be offset upwards.

Multiple stickily positioned elements in the same container are offset independently. Sticky position offsets may cause them to overlap.

Does the margin on the stickily positioned element affect its distance from the flow root edge?

Sticky positioning should really be defined in terms of the nearest scrollable ancestor, but there is currently no such term defined elsewhere in CSS. CSSOM View refers to "scrolling boxes." CSS Overflow has yet to pull in the relevant text from CSS Box, and CSS Box has an old, confusing definition of "flow root" which is almost (but probably not quite) what we want here. This spec refers to "flow root," since that’s the closest thing currently specced somewhere, but this is not optimal.

The following example is the same as the previous one, but now it is explained:
EM { font-style: italic }

6.3. Absolute positioning

In the absolute positioning model, a box is explicitly offset with respect to its containing block. It is removed from the normal flow entirely (it has no impact on later siblings). An absolutely positioned box establishes a new containing block for normal flow children and absolutely (but not fixed or page) positioned descendants. However, the contents of an absolutely positioned element do not flow around any other boxes. They may obscure the contents of another box (or be obscured themselves), depending on the stack levels of the overlapping boxes.

References in this specification to an absolutely positioned element (or its box) imply that the element's position property has the value absolute or fixed.

6.4. Center positioning

In the center positioning model, a box is explicitly centered with respect to its containing block. It is removed from the normal flow entirely (it has no impact on later siblings). A center positioned box establishes a new containing block for normal flow children and absolutely (but not fixed or page) positioned descendants. However, the contents of an center positioned element do not flow around any other boxes. They may obscure the contents of another box (or be obscured themselves), depending on the stack levels of the overlapping boxes.

References in this specification to an center positioned element (or its box) imply that the element's position property has the value center.

6.5. Page positioning

In the page positioning model, a box is explicitly offset with respect to its containing block. It is removed from the normal flow entirely (it has no impact on later siblings). A page positioned box establishes a new containing block for normal flow children and absolutely (but not fixed or other page) positioned descendants. However, the contents of a page positioned element do not flow around any other boxes. They may obscure the contents of another box (or be obscured themselves), depending on the stack levels of the overlapping boxes.

For paged media, boxes with page positions are only generated on the initial page where the page position element exists. Boxes with page position that are larger than the page area are clipped and the remaining part of the box is placed on the following page.

References in this specification to a page positioned element (or its box) imply that the element's position property has the value page.

6.6. Fixed positioning

Fixed positioning is similar to absolute positioning. The only difference is that for a fixed positioned box, the containing block is established by the viewport. For continuous media, fixed boxes do not move when the document is scrolled. In this respect, they are similar to fixed background images. For paged media, boxes with fixed positions are repeated on every page. This is useful for placing, for instance, a signature at the bottom of each page. Boxes with fixed position that are larger than the page area are clipped. Parts of the fixed position box that are not visible in the initial containing block will not print.

Authors may use fixed positioning to create the following presentation:

Example of frame layout [D]    [S]

This might be achieved with the following HTML document and style rules:


<!DOCTYPE html>
<html>
  <head>
    <title>A frame document with CSS</title>
    <style type="text/css" media="screen">
      body { height: 8.5in } /* Required for percentage heights below */
      #header {
        position: fixed;
        width: 100%;
        height: 15%;
        top: 0;
        right: 0;
        bottom: auto;
        left: 0;
      }
      #sidebar {
        position: fixed;
        width: 10em;
        height: auto;
        top: 15%;
        right: auto;
        bottom: 100px;
        left: 0;
      }
      #main {
        position: fixed;
        width: auto;
        height: auto;
        top: 15%;
        right: 0;
        bottom: 100px;
        left: 10em;
      }
      #footer {
        position: fixed;
        width: 100%;
        height: 100px;
        top: auto;
        right: 0;
        bottom: 0;
        left: 0;
      }
    </style>
  </head>
  <body>
    <div id="header"> ...  </div>
    <div id="sidebar"> ...  </div>
    <div id="main"> ...  </div>
    <div id="footer"> ...  </div>
  </body>
</html>

6.7. Choosing a positioning scheme: position property

The position property determines which of the positioning algorithms is used to calculate the position of a box.

Name:position
Value:static | relative | absolute | sticky | center | page | fixed
Initial:static
Applies to:all elements except table-column-group and table-column
Inherited:no
Percentages:N/A
Media:visual
Computed value:specified value
Animatable:no
Canonical order:per grammar

The values of this property have the following meanings:

static
The box is a normal box, laid out according to the normal flow. The top, right, bottom, and left properties do not apply.
relative
The box's position is calculated according to the normal flow (this is called the position in normal flow). Then the box is offset relative to its normal position and in all cases, including table elements, does not affect the position of any following boxes. When a box B is relatively positioned, the position of the following box is calculated as though B were not offset. The effect of position: relative on table elements is defined as follows:
absolute
The box's position (and possibly size) is specified with the top, right, bottom, and left properties. These properties specify offsets with respect to the box's containing block. Absolutely positioned boxes are taken out of the normal flow. This means they have no impact on the layout of later siblings. Though absolutely positioned boxes may have margins, those margins do not collapse with any other margins.
sticky
The box's position is calculated according to the normal flow (this is called the position in normal flow). Then the box is offset relative to its flow root and containing block and in all cases, including table elements, does not affect the position of any following boxes. When a box B is stickily positioned, the position of the following box is calculated as though B were not offset. The effect of position: sticky on table elements is the same as for position: relative
center
The box's position (and possibly size) is specified with the top, right, bottom, and left properties. The box is vertically and horizontally centered within its containing block and these properties specify offsets with respect to the box's centered position within its containing block. Center positioned boxes are taken out of the normal flow. This means they have no impact on the layout of later siblings. Though center positioned boxes may have margins, those margins do not collapse with any other margins.
page
The box's position is calculated according to the "absolute" model.

As with the "absolute" model, the box's margins do not collapse with any other margins. In the case of the print media type, the box is rendered only on the initial page or region where the page positioned element originated. User agents may paginate the content of paged boxes.

Note, that CSS Regions are also initial containing blocks, in accordance with flow-into property of the CSS Regions Module [CSS3-REGIONS].

fixed
The box's position is calculated according to the "absolute" model, but in addition, the box is fixed with respect to some reference. As with the "absolute" model, the box's margins do not collapse with any other margins. In the case of handheld, projection, screen, tty, and tv media types, the box is fixed with respect to the viewport and does not move when scrolled. In the case of the print media type, the box is rendered on every page, and is fixed with respect to the page box, even if the page is seen through a viewport (in the case of a print-preview, for example). For other media types, the presentation is undefined. Authors may wish to specify fixed in a media-dependent way. For instance, an author may want a box to remain at the top of the viewport on the screen, but not at the top of each printed page. The two specifications may be separated by using an '@media' rule, as in:

@media screen {
    h1#first { position: fixed }
}
@media print {
    h1#first { position: static }
}
User agents must not paginate the content of fixed boxes.

Note that user agents may print invisible content in other ways. See "Content outside the page box".

In previous versions of CSS user agents may treat position fixed as static on the root element. This specification removes that option and it is now required that newer user agents treat fixed value on the root element as defined for all other elements.

6.8. Box offsets: top, right, bottom, left

An element is said to be positioned if its position property has a value other than static. Positioned elements generate positioned boxes, and may be laid out according to the following four physical properties:

Name:top
Value:auto | <length> | <percentage>
Initial:auto
Applies to:positioned elements
Inherited:no
Percentages:refer to height of containing block
Media:visual
Computed value:For position: relative, see Relative positioning.
For position: sticky, see Sticky positioning.
For position: static, auto.
Otherwise: if specified as a '<length>', the corresponding absolute length; if specified as a '<percentage>', the specified value; otherwise, auto.
Animatable:<length>, <percentage>
Canonical order:per grammar

This property specifies how far an absolutely positioned box's top margin edge is offset below the top edge of the box's containing block.

For relatively positioned boxes, the offset is with respect to the top edge of the box itself (i.e., the box is given a position in the normal flow, and then offset from that position according to this property).

For stickily positioned boxes, the value is used to compute the sticky-constraint rectangle as described in Sticky positioning.

For absolutely positioned and center positioned elements whose containing block is based on a block-level element, this property is an offset from the padding edge of that element.

For page positioned boxes, within paged media or regions, the offset is with respect to the top edge of the initial containing block; otherwise for page positioned boxes same as for absolutely positioned boxes.

Name:right
Value:auto | <length> | <percentage>
Initial:auto
Applies to:positioned elements
Inherited:no
Percentages:refer to height of containing block
Media:visual
Computed value:For position: relative, see Relative positioning.
For position: sticky, see Sticky positioning.
For position: static, auto.
Otherwise: if specified as a '<length>', the corresponding absolute length; if specified as a '<percentage>', the specified value; otherwise, auto.
Animatable:<length>, <percentage>
Canonical order:per grammar

Similar to top, but specifies how far a box's right margin edge is offset to the left of the right edge of the box's containing block.

For relatively positioned boxes, the offset is with respect to the right edge of the box itself (i.e., the box is given a position in the normal flow, and then offset from that position according to this property).

For stickily positioned boxes, the value is used to compute the sticky-constraint rectangle as described in Sticky positioning.

For absolutely positioned and center positioned elements whose containing block is based on a block-level element, this property is an offset from the padding edge of that element.

For page positioned boxes, within paged media or regions, the offset is with respect to the right edge of the initial containing block; otherwise for page positioned boxes same as for absolutely positioned boxes.

Name:bottom
Value:auto | <length> | <percentage>
Initial:auto
Applies to:positioned elements
Inherited:no
Percentages:refer to height of containing block
Media:visual
Computed value:For position: relative, see Relative positioning.
For position: sticky, see Sticky positioning.
For position: static, auto.
Otherwise: if specified as a '<length>', the corresponding absolute length; if specified as a '<percentage>', the specified value; otherwise, auto.
Animatable:<length>, <percentage>
Canonical order:per grammar

Similar to top, but specifies how far a box's bottom margin edge is offset above the bottom edge of the box's containing block.

For relatively positioned boxes, the offset is with respect to the bottom edge of the box itself (i.e., the box is given a position in the normal flow, and then offset from that position according to this property).

For stickily positioned boxes, the value is used to compute the sticky-constraint rectangle as described in Sticky positioning.

For absolutely positioned and center positioned elements whose containing block is based on a block-level element, this property is an offset from the padding edge of that element.

For page positioned boxes, within paged media or regions, the offset is with respect to the bottom edge of the initial containing block; otherwise for page positioned boxes same as for absolutely positioned boxes.

Name:left
Value:auto | <length> | <percentage>
Initial:auto
Applies to:positioned elements
Inherited:no
Percentages:refer to height of containing block
Media:visual
Computed value:For position: relative, see Relative positioning.
For position: sticky, see Sticky positioning.
For position: static, auto. Otherwise: if specified as a '<length>', the corresponding absolute length; if specified as a '<percentage>', the specified value; otherwise, auto.
Animatable:<length>, <percentage>
Canonical order:per grammar

Similar to top, but specifies how far a box's left margin edge is offset to the right of the left edge of the box's containing block.

For relatively positioned boxes, the offset is with respect to the left edge of the box itself (i.e., the box is given a position in the normal flow, and then offset from that position according to this property).

For stickily positioned boxes, the value is used to compute the sticky-constraint rectangle as described in Sticky positioning.

For absolutely positioned and center positioned elements whose containing block is based on a block-level element, this property is an offset from the padding edge of that element.

For page positioned boxes, within paged media or regions, the offset is with respect to the left edge of the initial containing block; otherwise for page positioned boxes same as for absolutely positioned boxes.

The values for the four properties have the following meanings:

<length>
The offset is a fixed distance from the reference edge. Negative values are allowed.
<percentage>
The offset is a percentage of the containing block's width (for left or right) or height (for top and bottom). For stickily positioned elements, the offset is a percentage of the flow root’s width (for left or right) or height (for top or bottom). Negative values are allowed.
auto
For non-replaced elements, the effect of this value depends on which of related properties have the value auto as well. See the sections on the width and height of absolutely positioned, non-replaced elements for details. For replaced elements, the effect of this value depends only on the intrinsic dimensions of the replaced content. See the sections on the width and height of absolutely positioned, replaced elements for details.

Note, for page positioned elements and fixed positioned elements using large values or negative values may easily move elements outside the viewport and make the contents unreachable through scrolling or other means. Authors should be aware that page postion and fixed position elements are always relative to the initial containing block.

6.9. Logical box offsets: offset-before, offset-end, offset-after and offset-start

Logical offset properties allow for offseting positioned boxes based on the writing-mode and direction properties. When both the physical property and equivalent logical property (based on writing-mode and direction) are specified the physical property computes to the computed value of the coresponding logical property.

Positioned elements generate positioned boxes, and may be laid out according to the following four logical properties taking into account the writing-mode and direction of the containing block:

Name:offset-before
Value:auto | <length> | <percentage>
Initial:auto
Applies to:positioned elements
Inherited:no
Percentages:refer to height of containing block
Media:visual
Computed value:For position: relative, see Relative positioning.
For position: sticky, see Sticky positioning.
For position: static, auto.
Otherwise: if specified as a '<length>', the corresponding absolute length; if specified as a '<percentage>', the specified value; otherwise, auto.
Animatable:<length>, <percentage>
Canonical order:per grammar

For an absolutely positioned box this property specifies how far the coresponding margin edge is offset from the coresponding physical reference edge of the box's containing block.

The partiucular physical reference edge that is used when offsetting is based on the writing-mode and direction properties.

The combination of the writing-mode and direction properties determine the appropriate physical reference edge for offsetting.

The table below shows logical offset properties (per writing-mode and direction) and the physical reference edge from which the offset will occur.

Edge
top offset-before offset-before offset-start offset-end offset-start offset-end
right offset-end offset-start offset-before offset-before offset-after offset-after
bottom offset-after offset-after offset-end offset-start offset-end offset-start
left offset-start offset-end offset-after offset-after offset-before offset-before
writing-mode / direction
horizontal-tb / ltr horizontal-tb / rtl vertical-rl / ltr vertical-rl / rtl vertical-lr / ltr vertical-lr / rtl

Relationship of physical offset edges and the logical properties per writing-mode and direction.

For relatively positioned boxes, the offset is with respect to the property's coresponding physical reference edge of the box itself (i.e., the box is given a position in the normal flow, and then offset from that position according to the property).

This needs to be defined for sticky positioning.

For absolutely positioned and center positioned elements whose containing block is based on a block-level element, this property is an offset from the coresponding padding edge of that element.

For page positioned boxes, within paged media or regions, the offset is with respect to the property's coresponding physical edge of the initial containing block; otherwise for page positioned boxes same as for absolutely positioned boxes.

Name:offset-end
Value:auto | <length> | <percentage>
Initial:auto
Applies to:positioned elements
Inherited:no
Percentages:refer to height of containing block
Media:visual
Computed value:For position: relative, see Relative positioning.
For position: sticky, see Sticky positioning.
For position: static, auto.
Otherwise: if specified as a '<length>', the corresponding absolute length; if specified as a '<percentage>', the specified value; otherwise, auto.
Animatable:<length>, <percentage>
Canonical order:per grammar

Similar to offset-before but refer to the table, "Figure 1", for the coresponding physical reference edge that is used for offseting based on writing-mode and direction.

Name:offset-after
Value:auto | <length> | <percentage>
Initial:auto
Applies to:positioned elements
Inherited:no
Percentages:refer to height of containing block
Media:visual
Computed value:For position: relative, see Relative positioning.
For position: sticky, see Sticky positioning.
For position: statick, auto. Otherwise: if specified as a '<length>', the corresponding absolute length; if specified as a '<percentage>', the specified value; otherwise, auto.
Animatable:<length>, <percentage>
Canonical order:per grammar

Similar to offset-before but refer to the table, "Figure 1", for the coresponding physical reference edge that is used for offseting based on writing-mode and direction.

Name:offset-start
Value:auto | <length> | <percentage>
Initial:auto
Applies to:positioned elements
Inherited:no
Percentages:refer to height of containing block
Media:visual
Computed value:as specified
Animatable:<length>, <percentage>
Computed value:For position: relative, see Relative positioning.
For position: sticky, see Sticky positioning.
For position: statick, auto. Otherwise: if specified as a '<length>', the corresponding absolute length; if specified as a '<percentage>', the specified value; otherwise, auto.
Canonical order:per grammar

Similar to offset-before but refer to the table, "Figure 1", for the coresponding physical reference edge that is used for offseting based on writing-mode and direction.

The values for the four properties have the following meanings:

<length>
The offset is a fixed distance from the reference edge. Negative values are allowed.
<percentage>
The offset is a percentage of the containing block's width (for left or right) or height (for top and bottom). For stickily positioned elements, the offset is a percentage of the flow root’s width (for left or right) or height (for top or bottom). Negative values are allowed.
auto
For non-replaced elements, the effect of this value depends on which of related properties have the value auto as well. See the sections on the width and height of absolutely positioned, non-replaced elements for details. For replaced elements, the effect of this value depends only on the intrinsic dimensions of the replaced content. See the sections on the width and height of absolutely positioned, replaced elements for details.

Note, for page positioned elements and fixed positioned elements using large values or negative values may easily move elements outside the viewport and make the contents unreachable through scrolling or other means. Authors should be aware that page postion and fixed position elements are always relative to the initial containing block.

7. Choosing a different containing block for positioning

7.1. Attachment properties: attachment-type, attachment-top, attachment-right, attachment-bottom, attachment-left, and attachment

Ideas have been raised to allow changing the positioning containing block or allowing positioning and sizing to use different containing blocks.

8. Sizing and positioning details

8.1. The width of absolute, page or fixed positioned, non-replaced elements

The constraint that determines the used values for these elements is:

'left' + 'margin-left' + 'border-left-width' + 'padding-left' + 'width' + 'padding-right' + 'border-right-width' + 'margin-right' + 'right' = width of containing block

If all three of left, width, and right are auto: First set any auto values for margin-left and margin-right to 0. Then, if the direction property of the element establishing the static-position containing block is ltr set left to the static position and apply rule number three below; otherwise, set right to the static-position and apply rule number one below.

If none of the three is auto: If both margin-left and margin-right are auto, solve the equation under the extra constraint that the two margins get equal values, unless this would make them negative, in which case when direction of the containing block is ltr (rtl), set margin-left (margin-right) to 0 and solve for margin-right (margin-left). If one of margin-left or margin-right is auto, solve the equation for that value. If the values are over-constrained, ignore the value for left (in case the direction property of the containing block is rtl) or right (in case direction is ltr) and solve for that value.

Otherwise, set auto values for margin-left and margin-right to 0, and pick one of the following six rules that apply.

  1. If left and width are auto and right is not auto, then the width is shrink-to-fit. Then solve for left.
  2. If left and right are auto and width is not auto, then if the direction property of the element establishing the static-position containing block is ltr set left to the static-position, otherwise set right to the static-position. Then solve for left (if 'direction is rtl) or right (if direction is ltr).
  3. If width and right are auto and left is not auto, then the width is shrink-to-fit. Then solve for right.
  4. If left is auto, width and right are not auto, then solve for left.
  5. If width is auto, left and right are not auto, then solve for width.
  6. If right is auto, left and width are not auto, then solve for right.

8.2. The width of a center positioned, non-replaced elements

The following constraints must hold among the used values of the other properties:

'left' + 'margin-left' + 'border-left-width' + 'padding-left' + 'width' + 'padding-right' + 'border-right-width' + 'margin-right' + 'right' = width of containing block

If all three of left, width, and right are auto: First set any auto values for margin-left and margin-right to 0, then the width calculated as shrink-to-fit, and finally apply rule number two below.

If none of the three is auto: If both margin-left and margin-right are auto, solve the equation under the extra constraint that the two margins get equal values, unless this would make them negative, in which case when direction of the containing block is ltr (rtl), set margin-left (margin-right) to 0 and solve for margin-right (margin-left). If one of margin-left or margin-right is auto, solve the equation for that value. If the values are over-constrained, ignore the values for left and right and solve the equation so both propeties get equal values. This horizontally centers the element with respect to the edges of the containing block.

Otherwise, set auto values for margin-left and margin-right to 0, and pick one of the following six rules that apply.

  1. If left and width are auto and right is not auto, then the width is shrink-to-fit. Then solve for left.
  2. If both left and right are auto, their used values are equal. This horizontally centers the element with respect to the edges of the containing block.
  3. If width and right are auto and left is not auto, then the width is shrink-to-fit. Then solve for right.
  4. If left is auto, width and right are not auto, then solve for left.
  5. If width is auto, left and right are not auto, then solve for width.
  6. If right is auto, left and width are not auto, then solve for right.

8.3. The width of absolute, page or fixed positioned, replaced elements

If height and width both have computed values of auto and the element also has an intrinsic width, then that intrinsic width is the used value of width.

If height and width both have computed values of auto and the element has no intrinsic width, but does have an intrinsic height and intrinsic ratio; or if width has a computed value of auto, height has some other computed value, and the element does have an intrinsic ratio; then the used value of width is:

(used height) * (intrinsic ratio)

If height and width both have computed values of auto, the element has an intrinsic ratio but no intrinsic height or width, and the containing block's width does not itself depend on the replaced element's width, then the used value of width is calculated from the constraint equation used for block-level, non-replaced elements in normal flow.

Otherwise, if width has a computed value of auto, and the element has an intrinsic width, then that intrinsic width is the used value of width.

Otherwise, if width has a computed value of auto, but none of the conditions above are met, and then the used value of width becomes 300px. If 300px is too wide to fit the device, user agents should use the width of the largest rectangle that has a 2:1 ratio and fits the device instead.

After establishing the width, in order to position the replaced element, apply the following rules as appropriate.

  1. If both left and right have the value auto, and if the direction property of the element establishing the static-position containing block is ltr, set left to the static position and solve for right; else if direction is rtl, set right to the static position and solve for left.
  2. If left is auto and right is not auto, replace any auto on margin-left or margin-right with 0, then solve for left.
  3. If right is auto and left is not auto, replace any auto on margin-left or margin-right with 0, then solve for right.
  4. If at this point both margin-left and margin-right are still auto, solve the equation under the extra constraint that the two margins must get equal values, unless this would make them negative, in which case when the direction of the containing block is ltr (rtl), set margin-left (margin-right) to 0 and solve for margin-right (margin-left).
  5. If at this point there is an auto remaining, solve the equation for that value.
  6. If at this point the values are over-constrained, ignore the value for either left (in case the direction property of the containing block is rtl) or right (in case direction is ltr) and solve for that value.

8.4. The width of a center positioned, replaced elements

The used value for width is determined as for "The width of absolute, page or fixed positioned, replaced elements".

Then the rules for "The width of a center positioned, non-replaced elements" are applied to determine the centering.

8.5. The height of absolute, page or fixed positioned, non-replaced elements

For absolutely positioned elements, the used values of the vertical dimensions must satisfy this constraint:

'top' + 'margin-top' + 'border-top-width' + 'padding-top' + 'height' + 'padding-bottom' + 'border-bottom-width' + 'margin-bottom' + 'bottom' = height of containing block

If all three of top, height, and bottom are auto: First set any auto values for margin-top and margin-bottom to 0, then set top to the static position, and finally apply rule number three below.

If none of the three are auto: If both margin-top and margin-bottom are auto, solve the equation under the extra constraint that the two margins get equal values. If one of margin-top or margin-bottom is auto, solve the equation for that value. If the values are over-constrained, ignore the value for bottom and solve for that value.

Otherwise, set auto values for margin-top and margin-bottom to 0, and pick one of the following six rules that apply.

  1. If top and height are auto and bottom is not auto, then the height is based on the Auto heights for block formatting context roots, and solve for top.
  2. If top and bottom are auto and height is not auto, then set top to the static position, then solve for bottom.
  3. If height and bottom are auto and top is not auto, then the height is based on the Auto heights for block formatting context roots, and solve for bottom.
  4. If top is auto, height and bottom are not auto, then solve for top.
  5. If height is auto, top and bottom are not auto, then solve for height.
  6. If bottom is auto, top and height are not auto, then solve for bottom.

8.6. The height of a center positioned, non-replaced elements

The following constraints must hold among the used values of the other properties:

'top' + 'margin-top' + 'border-top-width' + 'padding-top' + 'height' + 'padding-bottom' + 'border-bottom-width' + 'margin-bottom' + 'bottom' = height of containing block

If all three of top, height, and bottom are auto: First set any auto values for margin-top and margin-bottom to 0, then the height is based on the Auto heights for block formatting context roots, and finally apply rule number two below.

If none of the three are auto: If both margin-top and margin-bottom are auto, solve the equation under the extra constraint that the two margins get equal values. If one of margin-top or margin-bottom is auto, solve the equation for that value. If the values are over-constrained, ignore the values for top and bottom and solve the equation so both propeties get equal values. This vertically centers the element with respect to the edges of the containing block.

Otherwise, set auto values for margin-top and margin-bottom to 0, and pick one of the following six rules that apply.

  1. If top and height are auto and bottom is not auto, then the height is based on the Auto heights for block formatting context roots, and solve for top.
  2. If top and bottom are auto and height is not auto, then set top and bottom to the have equal values. This vertically centers the element with respect to the edges of the containing block.
  3. If height and bottom are auto and top is not auto, then the height is based on the Auto heights for block formatting context roots, and solve for bottom.
  4. If top is auto, height and bottom are not auto, then solve for top.
  5. If height is auto, top and bottom are not auto, then solve for height.
  6. If bottom is auto, top and height are not auto, then solve for bottom.

8.7. The height of absolute, page or fixed positioned, replaced elements

If height and width both have computed values of auto and the element also has an intrinsic height, then that intrinsic height is the used value of height.

Otherwise, if height has a computed value of auto and the element has an intrinsic ratio then the used value of height is:

(used width) / (intrinsic ratio)

Otherwise, if height has a computed value of auto and the element has an intrinsic height, then that intrinsic height is the used value of height.

Otherwise, if height has a computed value of auto, but none of the conditions above are met, then the used value of height must be set to the height of the largest rectangle that has a 2:1 ratio, has a height not greater than 150px, and has a width not greater than the device width.

After establishing the height, in order to position the replaced element, apply the following rules as appropriate.

  1. If both top and bottom have the value auto, replace top with the element's static position.
  2. If bottom is auto, replace any auto on margin-top or margin-bottom with 0.
  3. If at this point both margin-top and margin-bottom are still auto, solve the equation under the extra constraint that the two margins must get equal values.
  4. If at this point there is only one auto remaining, solve the equation for that value.
  5. If at this point the values are over-constrained, ignore the value for bottom and solve for that value.

8.8. The height of a center positioned, replaced elements

The used value for height is determined as for "The height of absolute, page or fixed positioned, replaced elements".

Then the rules for "The height of a center positioned, non-replaced elements" are applied to determine the centering.

8.9. Auto heights for block formatting context roots

In certain cases (see, e.g., The height of absolute, page or fixed positioned, non-replaced element above), the height of an element that establishes a block formatting context is computed as follows:

If it only has inline-level children, the height is the distance between the top of the topmost line box and the bottom of the bottommost line box.

If it has block-level children, the height is the distance between the top margin-edge of the topmost block-level child box and the bottom margin-edge of the bottommost block-level child box.

Absolutely positioned children are ignored, and relatively positioned boxes are considered without their offset. Note that the child box may be an anonymous block box.

In addition, if the element has any floating descendants whose bottom margin edge is below the element's bottom content edge, then the height is increased to include those edges. Only non-positioned floats that participate in this block formatting context are taken into account, e.g., floats inside absolutely positioned descendants or other floats are not.

9. Relationships between display, position, and float

The three properties that affect box generation and layout — display, position, and float — interact as follows:

  1. If display has the value none, then position and float do not apply. In this case, the element generates no box.
  2. Otherwise, if position has the value absolute, page or fixed, and the value of float is left or right, the box is absolutely positioned and the computed value of float is none. The display is set according to the table below. Positioning of the box will determined by the top, right, bottom and left properties and the box's containing block.
  3. Otherwise, if float is other than none, the box is floated and display is set according to the table below.
  4. Otherwise, if the element is the root element, display is set according to the table below.
  5. Otherwise, the remaining display property values apply as specified.
Specified value Computed value
inline-table table
inline, table-row-group, table-column, table-column-group, table-header-group, table-footer-group, table-row, table-cell, table-caption, inline-block block
others same as specified

For new values defined for the display property, the respective modules that define a new value also define the handling of the relationship with positioning and floats.

10. Comparison of normal flow, floats, and positioning

This section is not normative.

To illustrate the differences between normal flow, relative positioning, floats, and absolute positioning, we provide a series of examples based on the following HTML:


<!DOCTYPE html>
<html>
    <head>
        <title>Comparison of positioning schemes</title>
    </head>
    <body>
    <p>
        Beginning of p contents.
        <span id="outer"> Start of outer contents.
        <span id="inner"> Inner contents.</span>
        End of outer contents.</span>
        End of p contents.
    </p>
    </body>
</html>

In this document, we assume the following rules:


body { display: block; font-size:12px; line-height: 200%;
        width: 400px; height: 400px }
p    { display: block }
span { display: inline }

The final positions of boxes generated by the outer and inner elements vary in each example. In each illustration, the numbers to the left of the illustration indicate the normal flow position of the double-spaced (for clarity) lines.

Note. The diagrams in this section are illustrative and not to scale. They are meant to highlight the differences between the various positioning schemes, and are not intended to be reference renderings of the examples given.

10.1. Normal flow

Consider the following CSS declarations for outer and inner that do not alter the normal flow of boxes:


#outer { color: red }
#inner { color: blue }

The P element contains all inline content: anonymous inline text and two SPAN elements. Therefore, all of the content will be laid out in an inline formatting context, within a containing block established by the P element, producing something like:

Image illustrating the normal flow of text between parent and sibling boxes.    [D]

10.2. Relative positioning

To see the effect of relative positioning, we specify:


#outer { position: relative; top: -12px; color: red }
#inner { position: relative; top: 12px; color: blue }

Text flows normally up to the outer element. The outer text is then flowed into its normal flow position and dimensions at the end of line 1. Then, the inline boxes containing the text (distributed over three lines) are shifted as a unit by -12px (upwards).

The contents of inner, as a child of outer, would normally flow immediately after the words "of outer contents" (on line 1.5). However, the inner contents are themselves offset relative to the outer contents by 12px (downwards), back to their original position on line 2.

Note that the content following outer is not affected by the relative positioning of outer.

Image illustrating the effects of relative positioning on a box’s content.    [D]

Note also that had the offset of outer been -24px, the text of outer and the body text would have overlapped.

10.3. Floating a box

Now consider the effect of floating the inner element's text to the right by means of the following rules:


#outer { color: red }
#inner { float: right; width: 130px; color: blue }

Text flows normally up to the inner box, which is pulled out of the flow and floated to the right margin (its width has been assigned explicitly). Line boxes to the left of the float are shortened, and the document's remaining text flows into them.

Image illustrating the effects of floating a box.    [D]

To show the effect of the clear property, we add a sibling element to the example:

<!DOCTYPE html>
<html>
    <head>
        <title>Comparison of positioning schemes II</title>
    </head>
    <body>
    <p>
        Beginning of p contents.
        <span id="outer"> Start of outer contents.
        <span id="inner"> Inner contents.</span>
        <span id="sibling"> Sibling contents.</span>
        End of outer contents.</span>
        End of p contents.
    </p>
    </body>
</html>

The following rules:


#inner { float: right; width: 130px; color: blue }
#sibling { color: red }

cause the inner box to float to the right as before and the document's remaining text to flow into the vacated space:

Image illustrating the effects of floating a box without setting the clear property to control the flow of text around the box.    [D]

However, if the clear property on the sibling element is set to right (i.e., the generated sibling box will not accept a position next to floating boxes to its right), the sibling content begins to flow below the float:


#inner { float: right; width: 130px; color: blue }
#sibling { clear: right; color: red }

Image illustrating the effects of floating
    an element with setting the clear property to control the flow of text around the
    element.    [D]

10.4. Absolute positioning

Next, we consider the effect of absolute positioning. Consider the following CSS declarations for outer and inner:


#outer {
    position: absolute;
    top: 200px; left: 200px;
    width: 200px;
    color: red;
}
#inner { color: blue }

which cause the top of the outer box to be positioned with respect to its containing block. The containing block for a positioned box is established by the nearest positioned ancestor (or, if none exists, the initial containing block, as in our example). The top side of the outer box is 200px below the top of the containing block and the left side is 200px from the left side. The child box of outer is flowed normally with respect to its parent.

Image illustrating the effects of absolutely positioning a box.    [D]

The following example shows an absolutely positioned box that is a child of a relatively positioned box. Although the parent outer box is not actually offset, setting its position property to relative means that its box may serve as the containing block for positioned descendants. Since the outer box is an inline box that is split across several lines, the first inline box's top and left edges (depicted by thick dashed lines in the illustration below) serve as references for top and left offsets.


#outer {
    position: relative;
    color: red
}
#inner {
    position: absolute;
    top: 200px; left: -100px;
    height: 130px; width: 130px;
    color: blue;
}

This results in something like the following:

Image illustrating the effects of absolutely positioning a box with respect to a containing block. [D]

If we do not position the outer box:


#outer { color: red }
#inner {
    position: absolute;
    top: 200px; left: -100px;
    height: 130px; width: 130px;
    color: blue;
}

the containing block for inner becomes the initial containing block (in our example). The following illustration shows where the inner box would end up in this case.

Image illustrating the effects of
      absolutely positioning a box with respect to a containing block established
      by a normally positioned parent.    [D]

Relative and absolute positioning may be used to implement change bars, as shown in the following example. The following fragment:


<p style="position: relative; margin-right: 10px; left: 10px;">
I used two red hyphens to serve as a change bar. They
will "float" to the left of the line containing THIS
<span style="position: absolute; top: auto; left: -1em; color: red;">--</span>
word.</p>

might result in something like:

Image illustrating the use of floats to create a changebar effect.    [D]

First, the paragraph (whose containing block sides are shown in the illustration) is flowed normally. Then it is offset 10px from the left edge of the containing block (thus, a right margin of 10px has been reserved in anticipation of the offset). The two hyphens acting as change bars are taken out of the flow and positioned at the current line (due to 'top: auto'), -1em from the left edge of its containing block (established by the P in its final position). The result is that the change bars seem to "float" to the left of the current line.

10.5. Page positioning

Finally, we consider the effect of page positioning. Consider the following CSS declarations for outer and inner:


#outer {
    position: page;
    top: 200px; left: 200px;
    width: 200px;
    color: red;
}
#inner { color: blue }

which cause the top of the outer box to be positioned with respect to its containing block. The containing block for a page positioned box is always established by the initial containing block. The top side of the outer box is 200px below the top of the initial containing block and the left side is 200px from the left side. The child box of outer is flowed normally with respect to its parent.

Image illustrating the effects of page positioning a box.    [D]

The following example shows a page positioned box that is a child of a relatively positioned box. Although the parent outer box is setting its position property to relative this outer box does not serve as the containing block for page positioned descendants. Since the inner box is a page positioned element its containing block is not the relative positioned outer box, page positioned elements are positioned from the initial containing block, in this case the top and left edges of the illustration itself.


#outer {
    position: relative;
    color: red
}
#inner {
    position: page;
    top: 200px; left: -100px;
    height: 130px; width: 130px;
    color: blue;
}

This results in something like the following:

Image illustrating the effects of
      page positioning a box with respect to the initial containing block.    [D]

Need to add example for center positioning.

11. Layered presentation

In the following sections, the expression "in front of" means closer to the user as the user faces the screen.

In CSS, each box has a position in three dimensions. In addition to their horizontal and vertical positions, boxes lie along a "z-axis" and are formatted one on top of the other. Z-axis positions are particularly relevant when boxes overlap visually. This section discusses how boxes may be positioned along the z-axis.

Each box belongs to one stacking context. Each box in a given stacking context has an integer stack level, which is its position on the z-axis relative to other boxes in the same stacking context. Boxes with greater stack levels are always formatted in front of boxes with lower stack levels. Boxes may have negative stack levels. Boxes with the same stack level in a stacking context are stacked bottom-to-top according to document tree order.

The root element creates a root stacking context, but other elements may establish local stacking contexts. Stacking contexts are inherited. A local stacking context is atomic; boxes in other stacking contexts may not come between any of its boxes.

An element that establishes a local stacking context generates a box that has two stack levels: one for the stacking context it creates (always 0) and one for the stacking context to which it belongs (given by the z-index property).

An element's box has the same stack level as its parent's box unless given a different stack level with the z-index property.

Name:z-index
Value:auto | <integer>
Initial:auto
Applies to:positioned elements
Inherited:no
Percentages:N/A
Media:visual
Computed value:as specified
Animatable:<integer>
Canonical order:per grammar

For a positioned box, the z-index property specifies:

  1. The stack level of the box in the current stacking context.
  2. Whether the box establishes a stacking context.

Values have the following meanings:

<integer>
This integer is the stack level of the generated box in the current stacking context. The box also establishes a new stacking context.
auto
The stack level of the generated box in the current stacking context is 0. The box does not establish a new stacking context unless it is the root element.

In the following example, the stack levels of the boxes (named with their "id" attributes) are: "text2"=0, "image"=1, "text3"=2, and "text1"=3. The "text2" stack level is inherited from the root box. The others are specified with the z-index property.


<!DOCTYPE html>
<html>
    <head>
        <title>Z-order positioning</title>
        <style type="text/css">
            .pile {
            position: absolute;
            left: 2in;
            top: 2in;
            width: 3in;
            height: 3in;
            }
        </style>
    </head>
    <body>
        <p>
            <img id="image" class="pile"
                src="butterfly.png" alt="A butterfly image"
                style="z-index: 1">
        <div id="text1" class="pile"
                style="z-index: 3">
            This text will overlay the butterfly image.
        </div>
        <div id="text2">
            This text will be beneath everything.
        </div>
        <div id="text3" class="pile"
                style="z-index: 2">
            This text will underlay text1, but overlay the butterfly image
        </div>
    </body>
</html>

This example demonstrates the notion of transparency. The default behavior of the background is to allow boxes behind it to be visible. In the example, each box transparently overlays the boxes below it. This behavior can be overridden by using one of the existing background properties.

12. Detailed stacking context

12.1. Definitions

Tree Order
The preorder depth-first traversal of the rendering tree, in logical (not visual) order for bidirectional content, after taking into account properties that move boxes around.
Element
In this description, "element" refers to actual elements, pseudo-elements, and anonymous boxes. Pseudo-elements and anonymous boxes are treated as descendants in the appropriate places. For example, an outside list marker comes before an adjoining '::before' box in the line box, which comes before the content of the box, and so forth.

12.2. Painting order

The bottom of the stack is the furthest from the user, the top of the stack is the nearest to the user:

A stacking context with four layers [D]

Schematic diagram of a stacking context with four layers.

The stacking context background and most negative positioned stacking contexts are at the bottom of the stack, while the most positive positioned stacking contexts are at the top of the stack.

The canvas is transparent if contained within another, and given a UA-defined color if it is not. It is infinite in extent and contains the root element. Initially, the viewport is anchored with its top left corner at the canvas origin.

The painting order for the descendants of an element generating a stacking context (see the z-index property) is:

  1. If the element is a root element:
    1. background color of element over the entire canvas.
    2. background image of element, over the entire canvas, anchored at the origin that would be used if it was painted for the root element.
  2. If the element is a block, list-item, or other block equivalent:
    1. background color of element unless it is the root element.
    2. background image of element unless it is the root element.
    3. column rule of the element.
    4. border of element.
    Otherwise, if the element is a block-level table:
    1. table backgrounds (color then image) unless it is the root element.
    2. column group backgrounds (color then image).
    3. column backgrounds (color then image).
    4. row group backgrounds (color then image).
    5. row backgrounds (color then image).
    6. cell backgrounds (color then image).
    7. cell column rule for multi-column.
    8. all table borders (in tree order for separated borders).
  3. Stacking contexts formed by positioned descendants with negative z-indices (excluding 0) in z-index order (most negative first) then tree order.
  4. For all its in-flow, non-positioned, block-level descendants in tree order: If the element is a block, list-item, or other block equivalent:
    1. background color of element.
    2. background image of element.
    3. column rule of the element.
    4. border of element.
    Otherwise, the element is a table:
    1. table backgrounds (color then image).
    2. column group backgrounds (color then image).
    3. column backgrounds (color then image).
    4. row group backgrounds (color then image).
    5. row backgrounds (color then image).
    6. cell backgrounds (color then image).
    7. cell column rule (multi-column).
    8. all table borders (in tree order for separated borders).
  5. All non-positioned floating descendants, in tree order. For each one of these, treat the element as if it created a new stacking context, but any positioned descendants and descendants which actually create a new stacking context are considered part of the parent stacking context, not this new one.
  6. If the element is an inline element that generates a stacking context, then:
    1. For each line box that the element is in:
      1. Jump to 7.2.1 for the box(es) of the element in that line box (in tree order).
  7. Otherwise: first for the element, then for all its in-flow, non-positioned, block-level descendants in tree order:
    1. If the element is a block-level replaced element, then: the replaced content, atomically.
    2. Otherwise, for each line box of that element:
      1. For each box that is a child of that element, in that line box, in tree order:
        1. background color of element.
        2. background image of element.
        3. column rule of the element.
        4. border of element.
        5. For inline elements:
          1. For all the elements in-flow, non-positioned, inline-level children that are in this line box, and all runs of text inside the element that is on this line box, in tree order:
            1. If this is a run of text, then:
              1. any underlining affecting the text of the element, in tree order of the elements applying the underlining (such that the deepest element's underlining, if any, is painted topmost and the root element's underlining, if any, is drawn bottommost).
              2. any overlining affecting the text of the element, in tree order of the elements applying the overlining (such that the deepest element's overlining, if any, is painted topmost and the root element's overlining, if any, is drawn bottommost).
              3. the text
              4. any line-through affecting the text of the element, in tree order of the elements applying the line-through (such that the deepest element's line-through, if any, is painted topmost and the root element's line-through, if any, is drawn bottommost).
            2. Otherwise, jump to 7.2.1 for that element
        6. For inline-block and inline-table elements:
          1. For each one of these, treat the element as if it created a new stacking context, but any positioned descendants and descendants which actually create a new stacking context are considered part of the parent stacking context, not this new one.
        7. For inline-level replaced elements:
          1. the replaced content, atomically.
        8. Optionally, the outline of the element (see 10 below).

        Note, some of the boxes may have been generated by line splitting or the Unicode bidirectional algorithm.

    3. Optionally, if the element is block-level, the outline of the element (see 10 below).
  8. All positioned, opacity or transform descendants, in tree order that fall into the following categories:
    1. All positioned descendants with 'z-index: auto' or 'z-index: 0', in tree order.

      For those with 'z-index: auto', treat the element as if it created a new stacking context, but any positioned descendants and descendants which actually create a new stacking context should be considered part of the parent stacking context, not this new one.

      For those with 'z-index: 0' treat the stacking context generated atomically.

    2. All opacity descendants with opacity less than 1, in tree order, create a stacking context generated atomically.
    3. All transform descendants with transform other than none, in tree order, create a stacking context generated atomically.
  9. Stacking contexts formed by positioned descendants with z-indices greater than or equal to 1 in z-index order (smallest first) then tree order.
  10. Finally, implementations that do not draw outlines in steps above must draw outlines from this stacking context at this stage. (It is recommended to draw outlines in this step and not in the steps above.)

12.3. Notes

The background of the root element is only painted once, over the whole canvas.

While the backgrounds of bidirectional inlines are painted in tree order, they are positioned in visual order. Since the positioning of inline backgrounds is unspecified in CSS, the exact result of these two requirements is UA-defined. A future version of CSS may define this in more detail.

13. Acknowledgments

This module would not have been possible without input and support from many helpful people. Thanks to Bert Bos, Tantek Çelik, Anton Prowse, Rossen Atanassov, Chris Jones, John Jansen, Sylvain Galineau.

Conformance

Document conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words "for example" or are set apart from the normative text with class="example", like this:

This is an example of an informative example.

Informative notes begin with the word "Note" and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Advisements are normative sections styled to evoke special attention and are set apart from other normative text with <strong class="advisement">, like this: UAs MUST provide an accessible alternative.

Conformance classes

Conformance to this specification is defined for three conformance classes:

style sheet
A CSS style sheet.
renderer
A UA that interprets the semantics of a style sheet and renders documents that use them.
authoring tool
A UA that writes a style sheet.

A style sheet is conformant to this specification if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.

A renderer is conformant to this specification if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by this specification by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)

An authoring tool is conformant to this specification if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.

Partial implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported component values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.

Experimental implementations

To avoid clashes with future CSS features, the CSS2.1 specification reserves a prefixed syntax for proprietary and experimental extensions to CSS.

Prior to a specification reaching the Candidate Recommendation stage in the W3C process, all implementations of a CSS feature are considered experimental. The CSS Working Group recommends that implementations use a vendor-prefixed syntax for such features, including those in W3C Working Drafts. This avoids incompatibilities with future changes in the draft.

Non-experimental implementations

Once a specification reaches the Candidate Recommendation stage, non-experimental implementations are possible, and implementors should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec.

To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.

Further information on submitting testcases and implementation reports can be found from on the CSS Working Group’s website at http://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.

References

Normative References

[CSS21]
Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-CSS2-20110607
[CSS3-REGIONS]
Vincent Hardy; Rossen Atanassov; Alan Stearns. CSS Regions Module Level 1. 28 May 2013. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2013/WD-css3-regions-20130528/
[CSS3BOX]
Bert Bos. CSS basic box model. 9 August 2007. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2007/WD-css3-box-20070809
[CSS3VAL]
Håkon Wium Lie; Tab Atkins; Elika J. Etemad. CSS Values and Units Module Level 3. 30 July 2013. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2013/CR-css3-values-20130730/
[rfc2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: http://www.ietf.org/rfc/rfc2119.txt

Informative References

[CSS1]
Håkon Wium Lie; Bert Bos. Cascading Style Sheets (CSS1) Level 1 Specification. 11 April 2008. REC. URL: http://www.w3.org/TR/CSS1/

Index

Property Index

NameValueInitialApplies toInh.%agesMediaAnimatableCanonical orderComputed valueComputed value
positionstatic | relative | absolute | sticky | center | page | fixedstaticall elements except table-column-group and table-columnnoN/Avisualnoper grammarspecified value
topauto | <length> | <percentage>autopositioned elementsnorefer to height of containing blockvisual<length>, <percentage>per grammarFor position: relative, see Relative positioning. For position: sticky, see Sticky positioning. For position: static, auto. Otherwise: if specified as a '<length>', the corresponding absolute length; if specified as a '<percentage>', the specified value; otherwise, auto.
rightauto | <length> | <percentage>autopositioned elementsnorefer to height of containing blockvisual<length>, <percentage>per grammarFor position: relative, see Relative positioning. For position: sticky, see Sticky positioning. For position: static, auto. Otherwise: if specified as a '<length>', the corresponding absolute length; if specified as a '<percentage>', the specified value; otherwise, auto.
bottomauto | <length> | <percentage>autopositioned elementsnorefer to height of containing blockvisual<length>, <percentage>per grammarFor position: relative, see Relative positioning. For position: sticky, see Sticky positioning. For position: static, auto. Otherwise: if specified as a '<length>', the corresponding absolute length; if specified as a '<percentage>', the specified value; otherwise, auto.
leftauto | <length> | <percentage>autopositioned elementsnorefer to height of containing blockvisual<length>, <percentage>per grammarFor position: relative, see Relative positioning. For position: sticky, see Sticky positioning. For position: static, auto. Otherwise: if specified as a '<length>', the corresponding absolute length; if specified as a '<percentage>', the specified value; otherwise, auto.
offset-beforeauto | <length> | <percentage>autopositioned elementsnorefer to height of containing blockvisual<length>, <percentage>per grammarFor position: relative, see Relative positioning. For position: sticky, see Sticky positioning. For position: static, auto. Otherwise: if specified as a '<length>', the corresponding absolute length; if specified as a '<percentage>', the specified value; otherwise, auto.
offset-endauto | <length> | <percentage>autopositioned elementsnorefer to height of containing blockvisual<length>, <percentage>per grammarFor position: relative, see Relative positioning. For position: sticky, see Sticky positioning. For position: static, auto. Otherwise: if specified as a '<length>', the corresponding absolute length; if specified as a '<percentage>', the specified value; otherwise, auto.
offset-afterauto | <length> | <percentage>autopositioned elementsnorefer to height of containing blockvisual<length>, <percentage>per grammarFor position: relative, see Relative positioning. For position: sticky, see Sticky positioning. For position: statick, auto. Otherwise: if specified as a '<length>', the corresponding absolute length; if specified as a '<percentage>', the specified value; otherwise, auto.
offset-startauto | <length> | <percentage>autopositioned elementsnorefer to height of containing blockvisual<length>, <percentage>per grammaras specifiedFor position: relative, see Relative positioning. For position: sticky, see Sticky positioning. For position: statick, auto. Otherwise: if specified as a '<length>', the corresponding absolute length; if specified as a '<percentage>', the specified value; otherwise, auto.
z-indexauto | <integer>autopositioned elementsnoN/Avisual<integer>per grammaras specified

Issues Index

Describe which element font-size-relative units are resolved against
Say what happens if this rectangle is empty
Say what happens if it already overflows the containing block
Do marins collapse between the stickily positioned element and its containing block element?
Does the margin on the stickily positioned element affect its distance from the flow root edge?
Sticky positioning should really be defined in terms of the nearest scrollable ancestor, but there is currently no such term defined elsewhere in CSS. CSSOM View refers to "scrolling boxes." CSS Overflow has yet to pull in the relevant text from CSS Box, and CSS Box has an old, confusing definition of "flow root" which is almost (but probably not quite) what we want here. This spec refers to "flow root," since that’s the closest thing currently specced somewhere, but this is not optimal.
This needs to be defined for sticky positioning.
Ideas have been raised to allow changing the positioning containing block or allowing positioning and sizing to use different containing blocks.
Need to add example for center positioning.