CSS Backgrounds and Borders Module Level 4

Editor’s Draft, 14 March 2014

This version:
http://dev.w3.org/csswg/css-backgrounds-4/
Latest version:
http://www.w3.org/TR/css-backgrounds-4/
Editor’s Draft:
http://dev.w3.org/csswg/css-backgrounds-4/
Feedback:
www-style@w3.org with subject line “[css-backgrounds] … message topic …”(archives)
Test Suite:
None Yet
Editors:
(W3C)
Elika J. Etemad (Invited Expert)
Lea Verou (Invited Expert)

Abstract

This module contains the features of CSS relating to the borders and backgrounds of boxes on the page. It includes and extends the functionality of CSS Backgrounds and Borders Level 3. [CSS3BG] The main extensions compared to level 3 are shaping (corner-shape) and clipping borders (border-clip), logical background positions (background-position), and the extend ability of background-repeat. 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-backgrounds” in the subject, preferably like this: “[css-backgrounds] …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.

Table of Contents

1 Introduction

This module is currently maintained as a diff against Level 3. We will fold in the text once it’s all formatted up and in CR again, as this will reduce the effort of keeping them in sync (source diffs will be accurate in reflecting the differences).

2 Backgrounds

2.1 Background Positioning: the background-position property

Name:backround-position
Value:<position>#
Initial:top left
Applies to:all
Inherited:no
Media:visual
Computed value:A list, each item consisting of: a pair of offsets and an origin, each offset given as a combination of an absolute length and a percentage
Percentages:refer to size of background positioning area minus size of background image

See [CSS3BG] for definition.

Where

<position> = [
  [ left | center | right | top | bottom | <percentage> | <length> ]
|
  [ left | center | right | <percentage> | <length> ]
  [ top | center | bottom | <percentage> | <length> ]
|
  [ center | [ left | right ] [ <percentage> | <length> ]? ] &&
  [ center | [ top | bottom ] [ <percentage> | <length> ]? ]
]

Add logical keywords

3 Corners

3.1 Corner Sizing: the 'border-radius property

Name:border-radius
Value:[ <length> | <percentage> ]{1,4} [ / [ <length> | <percentage> ]{1,4} ]?
Initial:0
Applies to:all elements, except table element when border-collapse is collapse
Inherited:no
Media:visual
Computed value:specified value
Canonical order:n/a
Percentages:n/a
Animatable:no

See [CSS3BG].

3.2 Corner Shaping: the corner-shape property

Name:corner-shape
Value:[round | bevel | scoop | notch]{1,4}
Initial:round
Applies to:all elements, except table element when border-collapse is collapse
Inherited:no
Media:visual
Computed value:specified value
Canonical order:n/a
Percentages:n/a
Animatable:no

By default, non-zero border-radii define a quarter-ellipse that rounds the affected corners. However in some cases, other corner shapes are desired. The corner-shape property specifies a reinterpretation of the radii to define other corner shapes.

round
Border radii define a convex elliptical curve at the corner.
bevel
Border radii define a diagonal slice at the corner.
scoop
Border radii define a concave elliptical curve at the corner.
notch
Border radii define a concave rectangular notch at the corner.
For example, the following declarations create a right-pointing next button.
  a {
    border-radius: 0 2em 2em 0;
    corner-shape: bevel;
    padding: 0.5em 2em 0.5em 0.5em;

As a fallback in UAs that don’t support border-radius, the right side would be rounded rather than pointy.

For scoop and notch, how do the color/style/width transitions work? Do they transition once in the middle, or alternate to match the topness/sideness of that segment of the border?

Add a cubic-bezier() function for random other shapes? Or some other functions?

3.3 Corner Shape and Size: the corners shorthand

Name:corners
Value:<‘corner-shape’> || <‘border-radius’>
Initial:see individual properties
Applies to:see individual properties
Inherited:see individual properties
Media:visual
Computed value:see individual properties
Percentages:see individual properties

The corners shorthand sets corner-shape and border-radius in the same declaration. If either is omitted, it is reset to its initial value.

For example, the following declaration creates a diamond shape.
corners: bevel 50%;

In UAs that don’t support corner-shape, the declaration is ignored (falls back to a rectangle).

In this example, the first declaration creates tabs with vertical sides and rounded corners using border-radius, while the second example makes them trapezoid-shaped in UAs that support corners.
  border-radius: 0.25em 0.25em 0 0;
  corners: bevel 0.25em 0.25em 0 0 / 50% 50% 0 0;

4 Partial borders

CSS borders traditionally cover an entire border edge. Sometimes, however, it can be useful to hide some parts of the border.

Here are two proposals for doing this: the second one is from GCPM, the first one is an attempt to recast it more readably. The names are terrible, known problem, proposals accepted. There is a problem with conceiving this as clipping: if you have dotted borders, you want whole dots always, not parts of dots. So it should be a drawing limit, not a clip.

4.1 Partial Borders: the border-limit property

Name: border-limit
Value: all | [ sides | corners ] [ <length> | <percentage> ]? | [ top | right | bottom | left ] [ <length> | <percentage> ]
Initial: round
Applies to: all elements, except table element when border-collapse is collapse
Inherited: no
Percentages: relative to border-box
Media: visual
Computed value: as specified

By default, the entire border is drawn. However, border rendering can be limited to only part of a border. The keyword specifies which part, and the length or percentage specifies how much.

sides
The sides are drawn up to but not including the corners (as defined by the border radii). A length or percentage is measured from the center of each side: 50% draws the middle 50% of the border; by default the entire side is drawn.
corners
The corners are drawn plus the specified distance into the sides if specified. A length is measured from the closest edge of the corner area. A percentage is measured from the absolute corner of the border box.
left
right
For the left and right (vertical) sides, draws the entire side and corner. For the top and bottom (horizontal) sides, draws the left/right portion, as specified. Distances are measured as for corners.
top
bottom
For the top and bottom (horizontal) sides, draws the entire side and corner. For the left and right (vertical) sides, draws the top/bottom portion, as specified. Distances are measured as for corners.

The following example draws only the middle 50% of the sides.

box { border: solid; border-parts: sides 50% }

The following example draws only the curved parts of the corners.

box { border: solid; border-radius: 1em 2em; border-parts: corners; }

The following example draws only the left 4em of the top border.

box { border-top: solid; border-parts: left 4em; }

The following example draws only the first 10px of each corner:

box { border: solid; border-parts: corners 10px; }

The following example draws the curved part of the corner plus 5px along ths sides:

box { border: solid; border-radius: 5px; border-shape: scoop; border-parts: corners 5px; }

The following example draws the curved part of the corner and all of the side except the middle 40%.

box { border: solid; border-radius: 5px; border-shape: scoope; border-parts: corners 30%; }

4.2 The border-clip properties

Name: border-clip, border-clip-top, border-clip-right, border-clip-bottom, border-clip-left
Value: normal | [ <length> | <percentage> | <fraction> ]+
Initial: normal
Applies to: all elements
Inherited: no
Percentages: refer to length of border-edge side
Media: visual
Computed value: normal, or a list consisting of absolute lengths, or percentages as specified

Should these properties be simplified to only accept normal | [ <length> | <percentage> ] +?

These properties split their respective borders into parts along the border edge. The first part is visible, the second is invisible, the third part is visible, etc. Parts can be specified with lengths, percentages, or fractions (expressed by the fr unit, as per [CSS3GRID] or its editor’s edition). The normal value means that the border is not split, but shown normally.

border-clip is a shorthand property for the four individual properties.

If the listed parts are shorter than the border, any remaining border is split proportionally between the specified fractions. If there are no fractions, the behavior is as if 1fr had been specified at the end of the list.

If the listed parts are longer than the border, the specified parts will be shown in full until the end of the border. In this case, all fractions will be zero.

For horizontal borders, parts are listed from left to right. For vertical borders, parts are listed from top to bottom.

The exact border parts are determined by laying out the specified border parts with all fractions initially set to zero. Any remaining border is split proportionally between the fractions specified.

border-clip: 10px 1fr 10px;
  border-clip-top: 10px 1fr 10px;
  border-clip-bottom: 10px 1fr 10px;
  border-clip-right: 5px 1fr 5px;
  border-clip-left: 5px 1fr 5px;

By making the first part have zero length, the inverse border of the previous example can easily be created:

  border-clip-top: 0 10px 1fr 10px;
  border-clip-bottom: 0 10px 1fr 10px;
  border-clip-right: 0 5px 1fr 5px;
  border-clip-left: 0 5px 1fr 5px;
  border: thin solid black;
  border-clip: 0 1fr; /* hide borders */
  border-clip-top: 10px 1fr 10px; /* make certain borders visible */
  border-clip-bottom: 10px 1fr 10px;
  border-top: thin solid black;
  border-bottom: thin solid black;
  border-clip-top: 10px;
  border-clip-bottom: 10px;
  
  border-top: thin solid black;
  border-clip: 10px;

This rendering:

  A sentence consists of words¹.
  ¹ Most often.
can be achieved with this style sheet:
  @footnote {
    border-top: thin solid black;
    border-clip: 4em;
  }
  border: 2px solid black;
  border-top-parts: repeat(10px 10px);

In this example, the repeat pattern is shown five times and there is, by coincidence, no remaining border.

  border: 2px solid black;
  border-top-parts: repeat(10px 10px);

In this example, the repeat pattern is shown five times. The box in this example is slightly wider than the box in the previous example. The remaining border is taken up by a fraction, as if this code had been specified:

  border: 2px solid black;
  border-top-parts: repeat(10px 10px) 1fr;

The fragment is shown in red for illustrative purposes; it should be shown in black by a compliant UA.

  border: 4px solid black;
  border-top-parts: 40px 20px 0 1fr repeat(20px 20px) 0 1fr 40px;

In this example, there will be a visible 40px border part on each end of the top border. Inside the 40px border parts, there will be an invisible border part of at least 20px. Inside these invisible border parts, there will be visible border parts, each 20px long with 20px invisible border parts between them.

The fragments are shown in red for illustrative purposes; they should not be visible in compliant UAs.

  border: 4px solid black;
  border-top-parts: 40px 20px 0 1fr 20px 20px 0 1fr 40px;

In this example, there will be a visible 40px border part on each end of the top border. Inside the 40px border parts, there will be an invisible border part of at least 20px. Inside these invisible border parts, there will be visible border parts, each 20px long with 20px invisible border parts between them.

The fragments are shown in red for illustrative purposes; they should not be visible in compliant UAs.

  border: 4px solid black;
  border-clip-top: 3fr 10px 2fr 10px 1fr 10px 10px 10px 1fr 10px 2fr 10px 3fr;

All but one of the visible border parts are represented as fractions in this example. The length of these border parts will change when the width of the element changes. Here is one rendering where 1fr ends up being 10px:

Here is another rendering where 1fr ends up being 30px:

The fragments are shown in red for illustrative purposes; they should be black in compliant UAs.

5 Changes

5.1 Additions Since Level 3

6 Acknowledgments

In addition to the many contributors to the [CSS1], [CSS21], and [CSS3BG] predecessors to this module, the editors would like to thank Tab Atkins, and Håkon Wium Lie for their suggestions and feedback specifically for this Level 4.

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.

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

[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. 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. W3C Recommendation. URL: http://www.w3.org/TR/2008/REC-CSS1-20080411
[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
[CSS3BG]
Bert Bos; Elika J. Etemad; Brad Kemper. CSS Backgrounds and Borders Module Level 3. 24 July 2012. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2012/CR-css3-background-20120724/
[CSS3GRID]
Alex Mogilevsky; Markus Mielke. CSS Grid Positioning Module Level 3. 5 September 2007. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2007/WD-css3-grid-20070905

Index

Property index

NameValueInitialApplies toInh.%agesMediaAnimatableCanonical orderComputed valueComputed value
backround-position<position>#top leftallnorefer to size of background positioning area minus size of background imagevisualA list, each item consisting of: a pair of offsets and an origin, each offset given as a combination of an absolute length and a percentage
border-radius[ <length> | <percentage> ]{1,4} [ / [ <length> | <percentage> ]{1,4} ]?0all elements, except table element when border-collapse is collapsenon/avisualnon/aspecified value
corner-shape[round | bevel | scoop | notch]{1,4}roundall elements, except table element when border-collapse is collapsenon/avisualnon/aspecified value
corners<‘corner-shape’> || <‘border-radius’>see individual propertiessee individual propertiessee individual propertiessee individual propertiesvisualsee individual properties
border-limitall | [ sides | corners ] [ <length> | <percentage> ]? | [ top | right | bottom | left ] [ <length> | <percentage> ]roundall elements, except table element when border-collapse is collapsenorelative to border-boxvisualas specified
border-clipnormal | [ <length> | <percentage> | <fraction> ]+normalall elementsnorefer to length of border-edge sidevisualnormal, or a list consisting of absolute lengths, or percentages as specified
border-clip-topnormal | [ <length> | <percentage> | <fraction> ]+normalall elementsnorefer to length of border-edge sidevisualnormal, or a list consisting of absolute lengths, or percentages as specified
border-clip-rightnormal | [ <length> | <percentage> | <fraction> ]+normalall elementsnorefer to length of border-edge sidevisualnormal, or a list consisting of absolute lengths, or percentages as specified
border-clip-bottomnormal | [ <length> | <percentage> | <fraction> ]+normalall elementsnorefer to length of border-edge sidevisualnormal, or a list consisting of absolute lengths, or percentages as specified
border-clip-leftnormal | [ <length> | <percentage> | <fraction> ]+normalall elementsnorefer to length of border-edge sidevisualnormal, or a list consisting of absolute lengths, or percentages as specified

Issues Index

This module is currently maintained as a diff against Level 3. We will fold in the text once it’s all formatted up and in CR again, as this will reduce the effort of keeping them in sync (source diffs will be accurate in reflecting the differences).
Add logical keywords
For scoop and notch, how do the color/style/width transitions work? Do they transition once in the middle, or alternate to match the topness/sideness of that segment of the border?
Add a cubic-bezier() function for random other shapes? Or some other functions?
Here are two proposals for doing this: the second one is from GCPM, the first one is an attempt to recast it more readably. The names are terrible, known problem, proposals accepted. There is a problem with conceiving this as clipping: if you have dotted borders, you want whole dots always, not parts of dots. So it should be a drawing limit, not a clip.
Should these properties be simplified to only accept normal | [ <length> | <percentage> ] +?