CSS Font Loading Module Level 3

Editor’s Draft, 17 October 2014

This version:
http://dev.w3.org/csswg/css-font-loading/
Latest version:
http://w3.org/TR/css-font-loading/
Previous Versions:
http://www.w3.org/TR/2014/WD-css-font-loading-3-20140522/
Feedback:
www-style@w3.org with subject line “[css-font-loading] … message topic …” (archives)
Editor:
Former Editor:
(Mozilla)

Abstract

This CSS module describes events and interfaces used for dynamically loading font resources.

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-font-loading” in the subject, preferably like this: “[css-font-loading] …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

CSS allows authors to load custom fonts from the web via the @font-face rule. While this is easy to use when authoring a stylesheet, it’s much more difficult to use dynamically via scripting.

Further, CSS allows the user agent to choose when to actually load a font; if a font face isn’t currently used by anything on a page, most user agents will not download its associated file. This means that later use of the font face will incur a delay as the user agent finally notices a usage and begins downloading and parsing the font file.

This specification defines a scripting interface to font faces in CSS, allowing font faces to be easily created and loaded from script. It also provides methods to track the loading status of an individual font, or of all the fonts on an entire page.

Several things in this spec use normal ES objects to define behavior, such as various things using Promises internally, and FontFaceSet using a Set internally. I believe the intention here is that these objects (and their prototype chains) are pristine, unaffected by anything the author has done. Is this a good intention? If so, how should I indicate this in the spec?

1.1. Values

This specification uses Promises, which are defined in ECMAScript 6. HTML5Rocks has some good tutorial material introducing Promises.

1.2. Task Sources

Whenever this specification queues a task, it queues it onto the "font loading" task source.

2. The FontFace Interface

The FontFace interface represents a single usable font face. CSS @font-face rules implicitly define FontFace objects, or they can be constructed manually from a url or binary data.

typedef (ArrayBuffer or ArrayBufferView) BinaryData;

dictionary FontFaceDescriptors {
  DOMString style = "normal";
  DOMString weight = "normal";
  DOMString stretch = "normal";
  DOMString unicodeRange = "U+0-10FFFF";
  DOMString variant = "normal";
  DOMString featureSettings = "normal";
};

enum FontFaceLoadStatus { "unloaded", "loading", "loaded", "error" };

[Constructor(DOMString family, (DOMString or BinaryData) source,
             optional FontFaceDescriptors descriptors),
 Exposed=Window,Worker]
interface FontFace {
  attribute DOMString family;
  attribute DOMString style;
  attribute DOMString weight;
  attribute DOMString stretch;
  attribute DOMString unicodeRange;
  attribute DOMString variant;
  attribute DOMString featureSettings;

  readonly attribute FontFaceLoadStatus status;

  Promise<FontFace> load();
  readonly attribute Promise<FontFace> loaded;
};

Other APIs use other names for .loaded, like .readyState. Can we align?

"unloaded" is usually used for things that start loaded and then become not loaded. Can we rename that value? Media stuff uses "none", other APIs use "empty", "idle", "nothing".

family, of type DOMString
style, of type DOMString
weight, of type DOMString
stretch, of type DOMString
unicodeRange, of type DOMString
These attributes all represent the corresponding aspects of a font face, as defined by the descriptors defined in the CSS @font-face rule. They are parsed the same as the corresponding @font-face descriptors. They are used by the font matching algorithm, but otherwise have no effect.

For example, a FontFace with a style of "italic" represents an italic font face; it does not make the font face italic.

On getting, return the string associated with this attribute.

On setting, parse the string according to the grammar for the CSS @font-face rule. If it does not match the grammar, throw a SyntaxError; otherwise, set the attribute to the serialization of the parsed value.

variant, of type DOMString
featureSettings, of type DOMString
These attributes have the same meaning, and are parsed the same as, the corresponding descriptors in the CSS @font-face rules.

They turn on or off specific features in fonts that support them. Unlike the previous attributes, these attributes actually affect the font face.

On getting, return the string associated with this attribute.

On setting, parse the string according to the grammar for the CSS @font-face rule. If it does not match the grammar, throw a SyntaxError; otherwise, set the attribute to the serialization of the parsed value.

status, of type FontFaceLoadStatus, readonly
This attribute reflects the current status of the font face. It must be "unloaded" for a newly-created FontFace.

It can change due to an author explicitly requesting a font face to load, such as through the load() method on FontFace, or implicitly by the user agent, due to it detecting that the font face is needed to draw some text on the screen.

loaded, of type Promise<FontFace>, readonly
This attribute reflects the [[FontStatusPromise]] of the font face.

All FontFace objects contain an internal [[FontStatusPromise]] slot, which tracks the status of the font. It starts out pending, and fulfills or rejects when the font is successfully loaded and parsed, or hits an error.

All FontFace objects also contain internal [[Urls]] and [[Data]] slots, of which one is null and the other is not null (the non-null one is set by the constructor, based on which data is passed in).

2.1. The Constructor

A FontFace can be constructed either from a URL pointing to a font face file, or from an ArrayBuffer (or ArrayBufferView) containing the binary representation of a font face.

When the FontFace(DOMString family, (DOMString or BinaryData) source, FontFaceDescriptors descriptors) method is called, execute these steps:

  1. Let font face be a fresh FontFace object. Set font face’s status attribute to "unloaded", Set its internal [[FontStatusPromise]] slot to a fresh pending Promise object.

    Parse the family argument, and the members of the descriptors argument, according to the grammars of the corresponding descriptors of the CSS @font-face rule. If any of them fail to parse correctly, reject font face’s [[FontStatusPromise]] with a DOMException named "SyntaxError", set font face’s corresponding attributes to the empty string, and set font face’s status attribute to "error". Otherwise, set font face’s corresponding attributes to the serialization of the parsed values.

    Return font face. If font face’s status is "error", terminate this algorithm; otherwise, complete the rest of these steps asynchronously.

  2. If the source argument was a DOMString, parse it according to the grammar of the src descriptor of the CSS @font-face rule. If it fails to parse correctly, reject font face’s [[FontStatusPromise]] with a DOMException named "SyntaxError" exception, queue a task to set font face’s status attribute to "error", and abort these steps; otherwise, set font face’s internal [[Urls]] slot to the string.

    Note: Note that this means that passing a naked url as the source argument, like "http://example.com/myFont.woff", won’t work - it needs to be at least wrapped in a url() function, like "url(http://example.com/myFont.woff)". In return for this inconvenience, you get to specify multiple fallbacks, specify the type of font each fallback is, and refer to local fonts easily.

    Need to define the base url, so relative urls can resolve. Should it be the url of the document? Is that correct for workers too, or should they use their worker url? Is that always defined?

    If the source argument was a BinaryData, set font face’s internal [[Data]] slot to the passed argument.

  3. If font face’s [[Data]] slot is not null, queue a task to set font face’s status attribute to "loading". Attempt to parse the data in it as a font. When this is completed, successfully or not, queue a task to run the following steps synchronously:

    1. If the load was successful, font face now represents the parsed font; fulfill font face’s [[FontStatusPromise]] with font face, and set its status attribute to "loaded".

    2. Otherwise, reject font face’s [[FontStatusPromise]] with a DOMException named "SyntaxError" and set font face’s status attribute to "error".

Note: Newly constructed FontFace objects are not automatically added to the FontFaceSet associated with a document or a context for a worker thread. This means that while newly constructed fonts can be preloaded, they cannot actually be used until they are explicitly added to a FontFaceSet. See the following section for a more complete description of FontFaceSet.

2.2. The load() method

The load() method of FontFace forces a url-based font face to request its font data and load. For fonts constructed from binary data, or fonts that are already loading or loaded, it does nothing.

When the load() method is called, execute these steps:

  1. Let font face be the FontFace object on which this method was called.
  2. If font face’s [[Urls]] slot is null, or its status attribute is anything other than "unloaded", return font face’s [[FontStatusPromise]] and abort these steps.
  3. Otherwise, set font face’s status attribute to "loading", return font face’s [[FontStatusPromise]], and continue executing the rest of this algorithm asynchronously.
  4. Using the value of font face’s [[Urls]] slot, attempt to load a font as defined in [CSS3-FONTS], as if it was the value of a @font-face rule’s src descriptor.
  5. When the load operation completes, successfully or not, queue a task to run the following steps synchronously:
    1. If the attempt to load fails, reject font face’s [[FontStatusPromise]] with a DOMException whose name is "NetworkError" and set font face’s status attribute to "error".
    2. Otherwise, font face now represents the loaded font; fulfill font face’s [[FontStatusPromise]] with font face and set font face’s status attribute to "loaded".

User agents can initiate font loads on their own, whenever they determine that a given font face is necessary to render something on the page. When this happens, they must act as if they had called the corresponding FontFace’s load() method described here.

Note: Some UAs utilize a "font cache" which avoids having to download the same font multiple times on a page or on multiple pages within the same origin. Multiple FontFace objects can be mapped to the same entry in the font cache, which means that a FontFace object might start loading unexpectedly, even if it’s not in a FontFaceSet, because some other FontFace object pointing to the same font data (perhaps on a different page entirely!) has been loaded.

2.3. Interaction with CSS’s @font-face Rule

A CSS @font-face rule automatically defines a corresponding FontFace object, which is automatically placed in the document’s font source when the rule is parsed. This FontFace object is CSS-connected.

The FontFace object corresponding to a @font-face rule has its family, style, weight, stretch, unicodeRange, variant, and featureSettings attributes set to the same value as the corresponding descriptors in the @font-face rule. There is a two-way connection between the two: any change made to a @font-face descriptor is immediately reflected in the corresponding FontFace attribute, and vice versa.

The internal [[Urls]] slot of the FontFace object is set to the value of the @font-face rule’s src descriptor, and reflects any changes made to the src descriptor.

Otherwise, a FontFace object created by a CSS @font-face rule is identical to one created manually.

If a @font-face rule is removed from the document, its corresponding FontFace object is no longer CSS-connected. The connection is not restorable by any means (but adding the @font-face back to the stylesheet will create a brand new FontFace object which is CSS-connected).

If a @font-face rule has its src descriptor changed to a new value, the original connected FontFace object must stop being CSS-connected. A new FontFace reflecting its new src must be created and CSS-connected to the @font-face. (This will also remove the old and add the new FontFace objects from any font sources they appear in.)

3. The FontFaceSet Interface

dictionary FontFaceSetLoadEventInit : EventInit {
  sequence<FontFace> fontfaces = [];
};

[Constructor(DOMString type, optional FontFaceSetLoadEventInit eventInitDict),
 Exposed=Window,Worker]
interface FontFaceSetLoadEvent : Event {
  readonly attribute sequence<FontFace> fontfaces;
};

enum FontFaceSetLoadStatus { "loading", "loaded" };

callback ForEachCallback = void (FontFace font, long index, FontFaceSet self);

[Exposed=Window,Worker,
 Constructor(sequence initialFaces)]
interface FontFaceSet : EventTarget {
  // Emulate the Set interface, until we can extend Set correctly.
  readonly attribute unsigned long size;
  void add(FontFace font);
  boolean has(FontFace font);
  boolean delete(FontFace font);
  void clear();
  Iterator entries();
  Iterator keys();
  Iterator values();
  void forEach(ForEachCallback cb, optional anythisArg);
  FontFace iterator;

  // -- events for when loading state changes
  attribute EventHandler onloading;
  attribute EventHandler onloadingdone;
  attribute EventHandler onloadingerror;

  // check and start loads if appropriate
  // and fulfill promise when all loads complete
  Promise<sequence<FontFace>> load(DOMString font, optional DOMString text = " ");

  // return whether all fonts in the fontlist are loaded
  // (does not initiate load if not available)
  boolean check(DOMString font, optional DOMString text = " ");

  // async notification that font loading and layout operations are done
  readonly attribute Promise<FontFaceSet> ready;

  // loading state, "loading" while one or more fonts loading, "loaded" otherwise
  readonly attribute FontFaceSetLoadStatus status;
};
ready of type Promise<FontFaceSet>, readonly
This attribute reflects the FontFaceSet's [[ReadyPromise]] slot.

See §3.4 The ready attribute for more details on this Promise and its use.

size of type unsigned long, readonly
This attribute reflects the size attribute of the FontFaceSet's [[ContainedFonts]] slot.
status of type FontFaceSetLoadStatus, readonly
If there are possibly pending font loads, the status attribute must have the value "loading". Otherwise, it must have the value "loaded".
FontFaceSet(sequence<FontFace> initialFaces)
The FontFaceSet constructor, when called, must construct a new Set object by passing its initialFaces argument to the Set constructor, then assigning that Set object to the FontFaceSet's [[ContainedFonts]] slot.
add(FontFace font)
has(FontFace font)
delete(FontFace font)
entries()
keys()
values()
forEach(ForEachCallback cb, optional any thisArg)
If add() or delete() are called with an argument that is a CSS-connected FontFace object, they must throw an InvalidModificationError exception.

All of these methods call the corresponding method of the FontFaceSet's [[ContainedFonts]] slot with the same arguments as were passed to them, and return the value returned by the called method.

clear()
This must remove all non-CSS-connected FontFace objects from the FontFaceSet's [[ContainedFonts]] slot.
iterator behavior
The iterator behavior is the iterator behavior of the FontFaceSet's [[ContainedFonts]] slot.

FontFaceSet objects also have internal [[LoadingFonts]], [[LoadedFonts]], and [[FailedFonts]] slots, all of which are initialized to the empty list, a [[ReadyPromise]] slot, which is initialized to a fresh pending Promise, and a [[ContainedFonts]] slot, which is initialized by the FontFaceSet constructor to a Set object (though see §4.2 Interaction with CSS’s @font-face Rule for information on how the Set may be pre-filled for FontFaceSet objects created by the user agent).

Because font families are loaded only when they are used, content sometimes needs to understand when the loading of fonts occurs. Authors can use the events and methods defined here to allow greater control over actions that are dependent upon the availability of specific fonts.

There are no pending font loads for a given FontFaceSet whenever none of its contained FontFace objects have a status of "loading".

For FontFaceSets that are font sources, in addition to the above constriant, the following must all be true for them to be considered as having no pending font loads:

If a FontFaceSet can’t be considered to have no pending font loads, it instead has possibly pending font loads.

3.1. Events

Font load events make it easy to respond to the font-loading behavior of the entire document, rather than having to listen to each font specifically. The loading event fires when the document begins loading fonts, while the loadingdone and loadingerror events fire when the document is done loading fonts, containing the fonts that successfully loaded or failed to load, respectively.

The following are the event handlers (and their corresponding event handler event types) that must be supported by FontFaceSet objects as IDL attributes:

Event handler Event handler event type
onloading loading
onloadingdone loadingdone
onloadingerror loadingerror

To fire a font load event named e at a FontFaceSet target with optional font faces means to fire a simple event named e using the FontFaceSetLoadEvent interface that also meets these conditions:

  1. The fontfaces attribute is initialized to the given list of FontFace objects.

Whenever one or more FontFace objects within a given FontFaceSet change their status attribute to "loading", the user agent must run the following steps:

  1. Let font face set be the given FontFaceSet, and loading fonts be the FontFace objects that have newly switched to "loading" status, in the same order as they appear in font face set.
  2. Set the status attribute of font face set to "loading".
  3. If font face set’s [[LoadingFonts]] slot is currently empty, fire a font load event named loading at font face set.
  4. Append the loading fonts to font face set’s [[LoadingFonts]] slot.
  5. If font face set’s [[ReadyPromise]] slot currently holds a fulfilled promise, replace it with a fresh pending promise.

Whenever one or more available font faces for a given FontFaceSet change their status attribute to "loaded" or "error", the user agent must run the following steps:

  1. Let font face set be the given FontFaceSet, and loaded fonts be the FontFace objects that have newly switched to "loaded" or "error" status, in the same order as they appear in font face set.
  2. For each font in the loaded fonts, if their status attribute is "loaded", append them to font face set’s [[LoadedFonts]] slot; if it’s "error", append them to font face set’s [[FailedFonts]] slot.

Whenever a FontFaceSet goes from having possibly pending font loads to having no pending font loads, user agents must run these steps:

  1. If this is not the first time the FontFaceSet has had no pending font loads, and none of its contained FontFace objects began loading since the last time it had no pending font loads, abort this algorithm.
  2. Set font face set’s status attribute to "loaded".
  3. Fire a font load event named loadingdone at font face set with the (possibly empty) contents of font face set’s [[LoadedFonts]] slot. Reset the [[LoadedFonts]] slot to an empty list.
  4. If font face set’s [[FailedFonts]] slots is non-empty, fire a font load event named loadingerror at font face set with the contents of font face set’s [[FailedFonts]] slot. Reset the [[FailedFonts]] slot to an empty list.
  5. Fulfill font face set’s [[ReadyPromise]] attribute’s value with font face set.

If asked to find the matching font faces from a FontFaceSet source, for a given font string font optionally some sample text text, and optionally an allow system fonts flag, run the following steps:

  1. Parse font using the CSS value syntax of the font property. If a syntax error occurs, return a syntax error.
  2. If text was not explicitly provided, let it be a string containing a single space character (U+0020 SPACE).
  3. Let font family list be the list of font families parsed from font, and font style be the other font style attributes parsed from font.
  4. Let available font faces be the font faces within source. If the allow system fonts flag is specified, add all system fonts to available font faces.
  5. Let matched font faces initially be an empty list.
  6. For each family in font family list, use the font matching rules to select the font faces from available font faces that match the font style, and add them to matched font faces. The use of the unicodeRange attribute means that this may be more than just a single font face.
  7. For each font face in matched font faces, if its defined unicode-range does not include the codepoint of at least one character in text, remove it from the list.
  8. Return matched font faces.

3.2. The load() method

The {{FontFaceSet/load() method of FontFaceSet will determine whether all fonts in the given font list have been loaded and are available. If any fonts are downloadable fonts and have not already been loaded, the user agent will initiate the load of each of these fonts. It returns a Promise, which is fulfilled when all of the fonts are loaded and ready to be used, or rejected if any font failed to load properly.

When the load(font, text) method is called, execute these steps:

  1. Let font face set be the FontFaceSet object this method was called on. Let promise be a newly-created promise object.
  2. Return promise. Complete the rest of these steps asynchronously.
  3. Find the matching font faces from font face set using the font and text arguments passed to the function, and let font face list be the return value. If a syntax error was returned, reject promise with a SyntaxError exception and terminate these steps.
  4. Queue a task to run the following steps synchronously:
    1. For all of the font faces in the font face list, call their load() method.
    2. Resolve promise with the result of waiting for all of the [[FontStatusPromise]]s of each font face in the font face list, in order.

3.3. The check() method

The check() method of FontFaceSet will determine whether all fonts in the given font list have been loaded and are available. If all fonts are available, it returns true; otherwise, it returns false.

When the check(font, text) method is called, execute these steps:

  1. Let font face set be the FontFaceSet object this method was called on.
  2. Find the matching font faces from font face set using the font and text arguments passed to the function, and including system fonts, and let font face list be the return value. If a syntax error was returned, throw a SyntaxError exception and terminate these steps.
  3. If the font face list contains no font faces, return false.
  4. If all fonts in the font face list have a status attribute of "loaded", or are system fonts, return true. Otherwise, return false.

3.4. The ready attribute

Because the number of fonts loaded depends on the how many fonts are used for a given piece of text, in some cases whether fonts need to be loaded or not may not be known. The ready attribute contains a Promise which is resolved when the document is done loading fonts, which provides a way for authors to avoid having to keep track of which fonts have or haven’t been loaded before examining content which may be affected by loading fonts.

Note: Authors should note that a given ready promise is only fulfilled once, but further fonts may be loaded after it fulfills. This is similar to listening for a loadingdone event to fire, but the callbacks passed to the ready promise will always get called, even when no font loads occur because the fonts in question are already loaded. It’s a simple, easy way to synchronize code to font loads without the need to keep track of what fonts are needed and precisely when they load.

Note: Note that the user agent may need to iterate over multiple font loads before the ready promise is fulfilled. This can occur with font fallback situations, where one font in the fontlist is loaded but doesn’t contain a particular glyph and other fonts in the fontlist need to be loaded. The ready promise is only fulfilled after layout operations complete and no additional font loads are necessary.

Note: Note that the Promise returned by this ready attribute is only ever fulfilled, never rejected, unlike the Promise returned by the FontFace load() method.

3.5. Interaction with CSS Font Loading and Matching

When the font matching algorithm in [CSS3-FONTS] is run automatically by the user-agent, the set of font faces it matches over must be precisely the set of fonts in the font source for the document, plus any local font faces.

When a user-agent needs to load a font face, it must do so by calling the load() method of the corresponding FontFace object.

(This means it must run the same algorithm, not literally call the value currently stored in the load property of the object.)

Fonts are available when they are added to a FontFaceSet. Adding a new @font-face rule to a stylesheet also adds a new FontFace to the FontFaceSet of the Document object.

Adding a new @font-face rule:

document.styleSheets[0].insertRule(
  "@font-face { font-family: newfont; src: url(newfont.woff); }", 0);
document.body.style.fontFamily = "newfont, serif";

Constructing a new FontFace object and adding it to document.fonts:

var f = new FontFace("newfont", "url(newfont.woff)");
document.fonts.add(f);
document.body.style.fontFamily = "newfont, serif";

In both cases, the loading of the font resource “newfont.woff” will be initiated by the layout engine, just as other @font-face rule fonts are loaded.

Omitting the addition to document.fonts means the font would never be loaded and text would be displayed in the default serif font:

var f = new FontFace("newfont", "url(newtest.woff)", {});

/* new {{FontFace}} not added to {{FontFaceSet}},
   so the 'font-family' property can’t see it,
   and serif will be used instead */
document.body.style.fontFamily = "newfont, serif";

To explicitly preload a font before using it, authors can defer the addition of a new FontFace to a FontFaceSet until the load has completed:

var f = new FontFace("newfont", "url(newfont.woff)", {});
f.load().then(function (loadedFace) {
  document.fonts.add(loadedFace);
  document.body.style.fontFamily = "newfont, serif";
});

In this case, the font resource “newfont.woff” is first downloaded. Once the download completes, the font is added to the document’s FontFaceSet, the body font is changed, and the layout engine uses the new font resource.

4. The FontFaceSource interface

[NoInterfaceObject]
interface FontFaceSource {
  readonly attribute FontFaceSet fonts;
};

Document implements FontFaceSource;
WorkerGlobalScope implements FontFaceSource;

Any document, workers, or other context which can use fonts in some manner must implement the FontFaceSource interface. The value of the context’s fonts attribute is its font source, which provides all of the fonts used in font-related operations, unless defined otherwise. Operations referring to “the font source” must be interpreted as referring to the font source of the relevant context in which the operation is taking place.

For any font-related operation that takes place within one of these contexts, the FontFace objects within the font source are its available font faces.

4.1. Worker FontFaceSources

Within a Worker document, the font source is initially empty.

Note: FontFace objects can be constructed and added to it as normal, which affects CSS font-matching within the worker (such as, for example, drawing text into a CanvasProxy).

4.2. Interaction with CSS’s @font-face Rule

The set entries for a document’s font source must be initially populated with all the CSS-connected FontFace objects from all of the CSS @font-face rules in the document’s stylesheets, in document order. As @font-face rules are added or removed from a stylesheet, or stylesheets containing @font-face rules are added or removed, the corresponding CSS-connected FontFace objects must be added or removed from the document’s font source, and maintain this ordering.

Note: Authors can still maintain references to a removed FontFace, even if it’s been automatically removed from a font source. As specified in §2.3 Interaction with CSS’s @font-face Rule, though, the FontFace is no longer CSS-connected at that point.

All non-CSS-connected FontFace objects must be sorted after the CSS-connected ones, in insertion order.

Note: It is expected that a future version of this specification will define ways of interacting with and querying local fonts as well.

5. API Examples

To show content only after all font loads complete:
document.fonts.ready().then(function() {
  var content = document.getElementById("content");
  content.style.visibility = "visible";
});
Drawing text in a canvas with a downloadable font, explicitly initiating the font download and drawing upon completion:
function drawStuff() {
  var ctx = document.getElementById("c").getContext("2d");

  ctx.fillStyle = "red";
  ctx.font = "50px MyDownloadableFont";
  ctx.fillText("Hello!", 100, 100);
}

document.fonts.load("50px MyDownloadableFont")
              .then(drawStuff, handleError);
A rich text editing application may need to measure text elements after editing operations have taken place. Since style changes may or may not require additional fonts to be downloaded, or the fonts may already have been downloaded, the measurement procedures need to occur after those font loads complete:
function measureTextElements() {
  // contents can now be measured using the metrics of
  // the downloadable font(s)
}

function doEditing() {
  // content/layout operations that may cause additional font loads
  document.fonts.ready().then(measureTextElements);
}
The loadingdone event only fires after all font related loads have completed and text has been laid out without causing additional font loads:
<style>
@font-face {
  font-family: latin-serif;
  src: url(latinserif.woff) format("woff"); /* contains no kanji/kana */
}
@font-face {
  font-family: jpn-mincho;
  src: url(mincho.woff) format("woff");
}
@font-face {
  font-family: unused;
  src: url(unused.woff);
}

body { font-family: latin-serif, jpn-mincho; }
</style>
<p>納豆はいかがでしょうか

In this situation, the user agent first downloads “latinserif.woff” and then tries to use this to draw the Japanese text. But because no Japanese glyphs are present in that font, fallback occurs and the font “mincho.woff” is downloaded. Only after the second font is downloaded and the Japanese text laid out does the loadingdone event fire.

The "unused" font isn’t loaded, but no text is using it, so the UA isn’t even trying to load it. It doesn’t interfere with the loadingdone event.

Changes

Changes from the May 2014 CSS Font Loading Last Call Working Draft:

  1. Corrected the async algorithms to use "queue a task" language, to ensure that side-effect timing is well-defined.

Acknowledgments

Several members of the Google Fonts team provided helpful feedback on font load events, as did Boris Zbarsky, Jonas Sicking and ms2ger.

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

[css3-fonts]
John Daggett. CSS Fonts Module Level 3. 11 December 2012. WD. URL: http://www.w3.org/TR/css3-fonts/
[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

Index

IDL Index

typedef (ArrayBuffer or ArrayBufferView) BinaryData;

dictionary FontFaceDescriptors {
  DOMString style = "normal";
  DOMString weight = "normal";
  DOMString stretch = "normal";
  DOMString unicodeRange = "U+0-10FFFF";
  DOMString variant = "normal";
  DOMString featureSettings = "normal";
};

enum FontFaceLoadStatus { "unloaded", "loading", "loaded", "error" };

[Constructor(DOMString family, (DOMString or BinaryData) source,
             optional FontFaceDescriptors descriptors),
 Exposed=Window,Worker]
interface FontFace {
  attribute DOMString family;
  attribute DOMString style;
  attribute DOMString weight;
  attribute DOMString stretch;
  attribute DOMString unicodeRange;
  attribute DOMString variant;
  attribute DOMString featureSettings;

  readonly attribute FontFaceLoadStatus status;

  Promise<FontFace> load();
  readonly attribute Promise<FontFace> loaded;
};

dictionary FontFaceSetLoadEventInit : EventInit {
  sequence<FontFace> fontfaces = [];
};

[Constructor(DOMString type, optional FontFaceSetLoadEventInit eventInitDict),
 Exposed=Window,Worker]
interface FontFaceSetLoadEvent : Event {
  readonly attribute sequence<FontFace> fontfaces;
};

enum FontFaceSetLoadStatus { "loading", "loaded" };

callback ForEachCallback = void (FontFace font, long index, FontFaceSet self);

[Exposed=Window,Worker,
 Constructor(sequence initialFaces)]
interface FontFaceSet : EventTarget {
  // Emulate the Set interface, until we can extend Set correctly.
  readonly attribute unsigned long size;
  void add(FontFace font);
  boolean has(FontFace font);
  boolean delete(FontFace font);
  void clear();
  Iterator entries();
  Iterator keys();
  Iterator values();
  void forEach(ForEachCallback cb, optional anythisArg);
  FontFace iterator;

  // -- events for when loading state changes
  attribute EventHandler onloading;
  attribute EventHandler onloadingdone;
  attribute EventHandler onloadingerror;

  // check and start loads if appropriate
  // and fulfill promise when all loads complete
  Promise<sequence<FontFace>> load(DOMString font, optional DOMString text = " ");

  // return whether all fonts in the fontlist are loaded
  // (does not initiate load if not available)
  boolean check(DOMString font, optional DOMString text = " ");

  // async notification that font loading and layout operations are done
  readonly attribute Promise<FontFaceSet> ready;

  // loading state, "loading" while one or more fonts loading, "loaded" otherwise
  readonly attribute FontFaceSetLoadStatus status;
};

[NoInterfaceObject]
interface FontFaceSource {
  readonly attribute FontFaceSet fonts;
};

Document implements FontFaceSource;
WorkerGlobalScope implements FontFaceSource;

Issues Index

Several things in this spec use normal ES objects to define behavior, such as various things using Promises internally, and FontFaceSet using a Set internally. I believe the intention here is that these objects (and their prototype chains) are pristine, unaffected by anything the author has done. Is this a good intention? If so, how should I indicate this in the spec?
Other APIs use other names for .loaded, like .readyState. Can we align?
"unloaded" is usually used for things that start loaded and then become not loaded. Can we rename that value? Media stuff uses "none", other APIs use "empty", "idle", "nothing".
Need to define the base url, so relative urls can resolve. Should it be the url of the document? Is that correct for workers too, or should they use their worker url? Is that always defined?