csswg-drafts/css-font-load-events/Overview.src.html at 017505aff76518b8f9f740fbb45705ea7a578bad · w3c/csswg-drafts · GitHub

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<h1>CSS Font Load Events Module Level 3</h1>

<pre class='metadata'>
Shortname: css-font-load-events
Level: 3
Group: csswg
Status: ED
ED: http://dev.w3.org/csswg/css-font-load-events/
TR: http://www.w3.org/TR/css-font-load-events-3/
Editor: Tab Atkins Jr., Google, http://xanthir.com/contact/
Former Editor: John Daggett, Mozilla, jdaggett@mozilla.com
Abstract: This CSS module describes events and interfaces used for dynamically loading font resources.
Link Defaults: css-fonts-3 (descriptor) src
Ignored Terms: promise, domstring, eventinit, cssfontfacerule, event, eventtarget, workerglobalscope
</pre>

<h2 id="introduction">
Introduction</h2>

	<p class='issue'>
		Fill in intro.

<h2 id="fontface-interface">
The <code>FontFace</code> Interface</h2>

	The <a interface>FontFace</a> 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.

	<pre class="idl">
		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,
			         FontFaceDescriptors descriptors)]
		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;

			// If the font face's source is url-based and its status isn't
			// "loading" or "loaded", this kicks off the load.
			void load();

			// Returns a promise that fulfills or rejects when the font face
			// reached "loaded" or "error" status.
			Promise ready();
		};
	</pre>

	<dl dfn-type=attribute dfn-for=FontFace>
		<dt><dfn>family</dfn>, of type <a interface>DOMString</a>
		<dt><dfn>style</dfn>, of type <a interface>DOMString</a>
		<dt><dfn>weight</dfn>, of type <a interface>DOMString</a>
		<dt><dfn>stretch</dfn>, of type <a interface>DOMString</a>
		<dt><dfn>unicodeRange</dfn>, of type <a interface>DOMString</a>
		<dd>
			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 <a interface>FontFace</a> with a <a attribute>style</a> of <code>"italic"</code>
			<em>represents</em> an italic font face;
			it does not <strong>make</strong> the font face italic.

		<dt><dfn>variant</dfn>, of type <a interface>DOMString</a>
		<dt><dfn>featureSettings</dfn>, of type <a interface>DOMString</a>
		<dd>
			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.

		<dt><dfn>status</dfn>, of type <a enum>FontFaceLoadStatus</a>
		<dd>
			This attribute reflects the current status of the font face.
			It must be "unloaded" for a newly-created <a interface>FontFace</a>.

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

	All <a interface>FontFace</a> objects contain an internal <dfn attribute for=FontFace>[FontStatusPromise]</dfn> attribute,
	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 <a interface>FontFace</a> objects also contain
	internal <dfn attribute for=FontFace>[Urls]</dfn> and <dfn attribute for=FontFace>[Data]</dfn> attributes,
	of which one is not <code>null</code> and the rest are <code>null</code>.

<h3 id='font-face-constructor'>
The Constructor</h3>

	A <a interface>FontFace</a> 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 <dfn method title='FontFace()' link-type=idl for=FontFace>Constructor(DOMString <dfn argument>family</dfn>, (DOMString or <a>BinaryData</a>) <dfn argument>source</dfn>, <a>FontFaceDescriptors</a> <dfn argument>descriptors</dfn>)</dfn> method is called,
	execute these steps:

	<ol link-type=idl>
		<li>
			Let <var>font face</var> be a fresh <a>FontFace</a> object.
			Set <var>font face's</var> <a for=FontFace>status</a> argument to <code>"unloaded"</code>.
			Set its internal <a>[FontStatusPromise]</a> attribute to a newly-created promise object.
			Return <var>font face</var>,
			and complete the rest of these steps asynchronously.

		<li>
			Parse the <a argument>family</a> argument,
			and the members of the <a argument for='FontFace()'>descriptors</a> argument,
			according to the grammars of the corresponding descriptors of the CSS ''@font-face'' rule.
			If any of them fail to parse correctly,
			reject <var>font face's</var> <a>[FontStatusPromise]</a> with a SyntaxError exception and abort these steps.
			Otherwise, set <var>font face's</var> corresponding attributes to the serialization of the parsed values.

		<li>
			If the <a>source</a> argument was a <a>DOMString</a>,
			parse it according to the grammar of the 'src' descriptor of the CSS ''@font-face'' rule.
			If it fails to parse correctly,
			reject <var>font face's</var> <a>[FontStatusPromise]</a> with a SyntaxError exception and abort these steps;
			otherwise, set <var>font face's</var> internal <a>[Urls]</a> attribute to the string.

			Note: Note that this means that passing a naked url as the source argument,
			like <code>"http://example.com/myFont.woff"</code>,
			won't work - it needs to be at least wrapped in a ''url()'' function,
			like <code>"url(http://example.com/myFont.woff)"</code>.
			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.

			If the <a>source</a> argument was a <a>BinaryData</a>,
			set <var>font face's</var> internal <a>[Data]</a> attribute to the passed argument.

		<li>
			If <var>font face's</var> <a>[Data]</a> attribute is not <code>null</code>,
			set <var>font face's</var> <a for=FontFace>status</a> attribute to "loading",
			and attempt to parse the data in it as a font.
			If this is successful,
			<var>font face</var> now represents the parsed font;
			fulfill <var>font face's</var> <a>[FontStatusPromise]</a> with <var>font face</var>,
			and set its <a for=FontFace>status</a> attribute to "loaded".
			If it is unsuccessful,
			reject <var>font face's</var> <a>[FontStatusPromise]</a> with a SyntaxError
			and set <var>font face's</var> <a for=FontFace>status</a> attribute to "error".
	</ol>

<h3 id='font-face-load'>
The <code>load()</code> method</h3>

	The <a method for=FontFace>load()</a> method of <a interface>FontFace</a>
	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 <dfn method for=FontFace>load()</dfn> method is called,
	execute these steps:

	<ol link-type=idl>
		<li>
			Let <var>font face</var> be the <a>FontFace</a> object on which this method was called.

		<li>
			If <var>font face's</var> <a>[Urls]</a> attribute is <code>null</code>,
			or its <a for=FontFace>status</a> attribute is anything other than <code>"unloaded"</code>,
			abort this algorithm immediately.

		<li>
			Otherwise,
			set <var>font face's</var> <a for=FontFace>status</a> attribute to "loading",
			return immediately,
			and follow these steps asynchronously:

		<li>
			Using the value of <var>font face's</a> <a>[Urls]</a> attribute,
			attempt to load a font as defined in [[!CSS3-FONTS]],
			as if it was the value of a ''@font-face'' rule's 'src' descriptor.

		<li>
			If the attempt to load fails,
			reject <var>font face's</var> <a>[FontStatusPromise]</a> with a NetworkError
			and set <var>font face's</var> <a for=FontFace>status</a> attribute to "error".

		<li>
			Otherwise,
			<var>font face</var> now represents the loaded font;
			fulfill <var>font face's</var> <a>[FontStatusPromise]</a> with <var>font face</var>
			and set <var>font face's</var> <a for=FontFace>status</a> attribute to "loaded".
	</ol>

	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 <a interface>FontFace</a>’s <a method for=FontFace>load()</a> method described here.

<h3 id='font-face-ready'>
The <code>ready()</code> method</h3>

	The <a method for=FontFace>ready()</a> method of <a interface>FontFace</a>
	returns a Promise
	which is fulfilled when the font face is successfully loaded,
	or rejected when the font face fails to load or parse successfully.

	When the <dfn method for="FontFace">ready()</dfn> method is called,
	execute these steps:

	<ol link-type=idl>
		<li>
			Let <var>font face</var> be the <a>FontFace</a> object on which this method was called.

		<li>
			Let <var>ready promise</var> be a newly-created promise object.

		<li>
			Resolve <var>ready promise</var> with <var>font face's</var> <a attribute>[FontStatusPromise]</a>.
		<li>
			Return <var>ready promise</var>.
	</ol>

<h3 id='font-face-css-connection'>
Interaction with CSS’s ''@font-face'' Rule</h3>

	A CSS ''@font-face'' rule automatically defines a corresponding <a interface>FontFace</a> object,
	which is automatically placed in the document's <a>font source</a>.
	This <a interface>FontFace</a> object is <dfn>CSS-connected</dfn>.

	The <a interface>FontFace</a> object corresponding to a ''@font-face'' rule
	has its <a attribute>family</a>, <a attribute>style</a>, <a attribute>weight</a>, <a attribute>stretch</a>, <a attribute>unicodeRange</a>, <a attribute>variant</a>, and <a attribute>featureSettings</a> 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 <a interface>FontFace</a> attribute,
	and vice versa.

	The internal <a attribute>[Urls]</a> attribute of the <a interface>FontFace</a> 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 <a interface>FontFace</a> 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 <a interface>FontFace</a> object is no longer <a>CSS-connected</a>.  The connection is not restorable by any means (but adding the ''@font-face'' back to the stylesheet will create a brand new <a interface>FontFace</a> object which <em>is</em> <a>CSS-connected</a>).


<h2 id="FontFaceSet-interface">
The <code>FontFaceSet</code> Interface</h2>

	<pre class="idl">
		dictionary CSSFontFaceLoadEventInit : EventInit {
			sequence&lt;CSSFontFaceRule> fontfaces = null;
		};

		[Constructor(DOMString type, optional CSSFontFaceLoadEventInit eventInitDict)]
		interface CSSFontFaceLoadEvent : Event {
			readonly attribute sequence&lt;CSSFontFaceRule> fontfaces;
		};

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

		[SetClass(FontFace)]
		interface FontFaceSet {

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

			// Return the fonts that match the given "font" argument
			// and can render the given characters.
			sequence&lt;FontFace> match(DOMString font, optional DOMString text = " ");

			// check and start loads if appropriate
			// and fulfill promise when all loads complete
			Promise 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
			Promise ready();

			// loading state, true while one or more fonts loading, false otherwise
			readonly attribute FontFaceLoadStatus status;
		};

		FontFaceSet implements EventTarget;
	</pre>

	The set entries for a <a interface>FontFaceSet</a> is initially an empty list.
	A <a interface>FontFaceSet</a> attached to a document may have some initial <a interface>FontFace</a> objects prefilled in it;
	see the section on <a href="#document-font-face-set">Interactions with CSS's @font-face Rule</a> for details.

	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.

	The term <dfn>font load</dfn> indicates when the
	loading of content for a given <a interface>FontFace</a> object completes.
	A <a interface>FontFace</a> object may list multiple alternate resources
	within its <a attribute>[Urls]</a> attribute,
	including references to local fonts,
	but the term font load only refers to the loading of the finally selected resource for a given <a interface>FontFace</a>,
	not to the loading of each individual resource.

	There are <dfn>no pending font loads</dfn> whenever all of the following are true:

	<ul>
		<li>
			The document has finished loading.

		<li>
			There are no pending stylesheet requests.

		<li>
			There are no <a>available font faces</a>
			with a <a attribute for=FontFace>status</a> of "loading".

		<li>
			There are no pending layout operations which might cause the user agent to request a font.
	</ul>

	If any of the above conditions are false,
	there are <dfn>possibly pending font loads</dfn>.

	<a interface>FontFaceSet</a> objects also have internal
	<dfn attribute for=FontFaceSet>[LoadingFonts]</dfn>,
	<dfn attribute for=FontFaceSet>[LoadedFonts]</dfn>,
	<dfn attribute for=FontFaceSet>[FailedFonts]</dfn>,
	and <dfn attribute for=FontFaceSet>[PendingReadyPromises]</dfn> attributes,
	all of which are initialized to the empty list.

	<p class="issue">
		Document the FontFaceSet behavior for Workers.
		(They start out empty, but can construct fonts or be sent them via postMessage.)

<h3 id='set-modifications'>
Modifications of normal Set methods</h3>

	The <a interface>FontFaceSet</a> methods <code>add()</code> and <code>delete()</code>
	must throw an InvalidModificationError exception
	if their argument is a <a>CSS-connected</a> <a interface>FontFace</a> object.

	The <a interface>FontFaceSet</a> method <code>clear()</code>
	must only remove the manually-added <a interface>FontFace</a> objects;
	the <a>CSS-connected</a> <a interface>FontFace</a> objects are unaffected.


<h3 id='FontFaceSet-events'>
Events</h3>

	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 <dfn event for="FontFaceSet">loading</dfn> event
	fires when the document begins loading fonts,
	while the <dfn event for="FontFaceSet">loadingdone</dfn>
	and <dfn event for="FontFaceSet">loadingerror</dfn> 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 <code>FontFaceSet</code> objects as IDL attributes:

	<table class="data" id="eventhandlers">
		<thead>
			<tr>
				<th>Event handler
				<th>Event handler event type
		<tbody>
			<tr>
				<th><a attribute>onloading</a>
				<td><a event>loading</a>
			<tr>
				<th><a attribute>onloadingdone</a>
				<td><a event>loadingdone</a>
			<tr>
				<th><a attribute>onloadingerror</a>
				<td><a event>loadingerror</a>
	</table>

	To <dfn>fire a font load event</dfn> named <var>e</var>
	at a <a interface>FontFaceSet</a> <var>target</var>
	with optional <var>font faces</var>
	means to
	<a href="http://www.w3.org/TR/html5/webappapis.html#event-firing">fire a simple event</a> named <var>e</var>
	using the <a interface>CSSFontFaceLoadEvent</a> interface that also meets these conditions:

	<ol>
		<li>
			The <a attribute>fontfaces</a> attribute is initialized to the given list of <a interface>FontFace</a> objects.
	</ol>

	Whenever a <a interface>FontFace</a> in a given <a interface>FontFaceSet</a> <var>font face set</var>
	changes its <a attribute for=FontFace>status</a> attribute to

	Whenever one or more <a>available font faces</a> for a given <a interface>FontFaceSet</a>
	change their <a attribute for=FontFace>status</a> attribute to "loading",
	the user agent must run the following steps:

	<ol>
		<li>
			Let <var>font face set</var> be the given <a interface>FontFaceSet</a>,
			and <var>loading fonts</var> be the <a interface>FontFace</a> objects
			that have newly switched to "loading" status,
			in the same order as they appear in <var>font face set</var>.

		<li>
			Set the <a attribute for=FontFaceSet>status</a> attribute of <var>font face set</var> to "loading".

		<li>
			If <var>font face set's</var> <a attribute>[LoadingFonts]</a> attribute is currently empty,
			<a>fire a font load event</a> named <a event>loading</a> at <var>font face set</var>.

		<li>
			Append the <var>loading fonts</var> to <var>font face set's</var> <a attribute>[LoadingFonts]</a> attribute.
	</ol>

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

	<ol>
		<li>
			Let <var>font face set</var> be the given <a interface>FontFaceSet</a>,
			and <var>loaded fonts</var> be the <a interface>FontFace</a> objects
			that have newly switched to "loaded" or "error" status,
			in the same order as they appear in <var>font face set</var>.

		<li>
			For each font in the <var>loaded fonts</var>,
			if their <a attribute for=FontFace>status</a> attribute is "loaded",
			append them to <var>font face set's</var> <a attribute>[LoadedFonts]</a> attribute;
			if it's "error",
			append them to <var>font face set's</var> <a attribute>[FailedFonts]</a> attribute.
	</ol>

	If there are ever <a>no pending font loads</a>
	and either of <var>font face set's</var> <a attribute>[LoadedFonts]</a> or <a attribute>[FailedFonts]</a> attributes are not empty,
	user agents must run these steps:

	<ol>
		<li>
			Set <var>font face set's</var> <a attribute for=FontFaceSet>status</a> attribute to "loaded".

		<li>
			<a>Fire a font load event</a> named <a event>loadingdone</a> at <var>font face set</var>
			with the contents of <var>font face set's</var> <a attribute>[LoadedFonts]</a> attribute, if any.

		<li>
			If <var>font face set's</var> <a attribute>[FailedFonts]</a> attribute is non-empty,
			<a>fire a font load event</a> named <a event>loadingerror</a> at <var>font face set</var>
			with the contents of <var>font face set's</var> <a attribute>[FailedFonts]</a> attribute.

		<li>
			If <var>font face set's</var> <a attribute>[PendingReadyPromises]</a> attribute is non-empty,
			fulfill each promise in the attribute's value with <var>font face set</var>.

		<li>
			Empty <var>font face set's</var> <a attribute>[LoadingFonts]</a>, <a attribute>[LoadedFonts]</a>, <a attribute>[FailedFonts]</a>, and <a attribute>[PendingReadyPromises]</a> attributes.
	</ol>


<h3 id='font-face-set-match'>
The <code>match()</code> method</h3>
	The <a for=FontFaceSet>match()</a> method of <a interface>FontFaceSet</a> returns
	the subset of <a>available font faces</a>
	that match a given 'font' specifier
	and can render the given characters.

	When the <dfn method for="FontFaceSet">match(<dfn argument>font</dfn>, <dfn argument>text</dfn>)</dfn> method is called,
	execute these steps:

	<ol>
		<li>
			Parse the value of the <a argument for='load()'>font</a> argument,
			using the CSS value syntax of the 'font' property.

		<li>
			If a syntax error occurs,
			throw a SyntaxError exception.

		<li>
			Otherwise, let <var>font family list</var> be the set of families and
			<var>font style</var> be the other font style attributes.

		<li>
			For each family in <var>font family list</var>,
			use the font matching rules to select the <a>available font faces</a> that match the <var>font style</var>.
			The use of the <a attribute>unicodeRange</a> attribute means that this may be more than just a single font face.

		<li>
			Remove from the set of font faces all faces that have defined 'unicode-range'
			values that don't intersect the range of character values in the <a argument for='load()'>text</a>
			argument and set this to be the <var>font load list</var>.

		<li>
			Return the <var>font load list</var>.
	</ol>


<h3 id='font-face-set-load'>
The <code>load()</code> method</h3>

	The <a for=FontFaceSet>load()</a> method of <a interface>FontFaceSet</a> 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 <dfn method for="FontFaceSet">load(<dfn argument>font</dfn>, <dfn argument>text</dfn>)</dfn> method is called,
	execute these steps:

	<ol link-type=idl>
		<li>
			Let <var>font face set</var> be the <a interface>FontFaceSet</a> object this method was called on.
			Let <var>promise</var> be a newly-created promise object.

		<li>
			Return <var>promise</var>.
			Complete the rest of these steps asynchronously.

		<li>
			Call <var>font face set's</var> <a method for=FontFaceSet>match()</a> method
			with the same arguments as this method,
			and let <var>font face list</var> be the return value.
			If <a method for=FontFaceSet>match()</a> threw an exception,
			reject <var>promise</var> with that same exception and terminate these steps.

		<li>
			For all of the font faces in the <var>font face list</var>,
			call their <a method for=FontFace>load()</a> method.

		<li>
			Resolve <var>promise</var> with the result of
			waiting for all of the <a>[FontStatusPromise]</a>s of each font face in the <var>font face list</var>, in order.
	</ol>

<h3 id='font-face-set-check'>
The <code>check()</code> method</h3>

	The <a>check()</a> method of <a interface>FontFaceSet</a> 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 <dfn method for="FontFaceSet">check(<dfn argument>font</dfn>, <dfn argument>text</dfn>)</dfn> method is called,
	execute these steps:

	<ol>
		<li>
			Let <var>font face set</var> be the <a interface>FontFaceSet</a> object this method was called on.

		<li>
			Call <var>font face set's</var> <a method for=FontFaceSet>match()</a> method
			with the same arguments as this method,
			and let <var>font face list</var> be the return value.
			If <a method for=FontFaceSet>match()</a> threw an exception,
			throw that same exception and terminate these steps.

		<li>
			If the <var>font face list</var> contains no font faces,
			return <code class="idl-code">false</code>.

		<li>
			If all fonts in the <var>font face list</var> have a <a attribute for=FontFace>status</a> attribute of "loaded",
			return <code>true</code>.
			Otherwise, return <code>false</code>.
	</ol>

<h3 id='font-face-set-ready'>
The <code>ready()</code> method</h3>

	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 <a for=FontFaceSet>ready()</a> method returns 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.

	When the <dfn method for=FontFaceSet>ready()</dfn> method is called,
	execute these steps:

	<ol>
		<li>
			Let <var>font face set</var> be the <a interface>FontFaceSet</a> object this method was called on.
			Let <var>ready promise</var> be a fresh Promise which is initially pending.

		<li>
			If <var>font face set's</var> <a attribute for=FontFaceSet>status</a> attribute is "loaded",
			fulfill <var>ready promise</var> with <var>font face set</var>.
			Otherwise, append <var>ready promise</var>
			to <var>font face set's</var> <a attribute>[PendingReadyPromises]</a> attribute.

		<li>
			Return <var>ready promise</var>.
	</ol>

	Note: Authors should note that a given <var>ready promise</var> is only fulfilled once,
	but further fonts may be loaded after it fulfills.
	This is similar to listening for a <a event>loadingdone</a> event to fire,
	but the callbacks passed to the <a method for=FontFaceSet>ready()</a> promise will <strong>always</strong> 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 <var>ready promise</var> 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 <var>ready promise</var> is only fulfilled after layout operations complete
	and no additional font loads are necessary.

	Note: Note that the Promise returned by this <a method for=FontFaceSet>ready()</a> method is only ever fulfilled,
	never rejected,
	unlike the Promise returned by the <a interface>FontFace</a> <a method for=FontFace>ready()</a> method.

<h3 id='font-face-set-css'>
Interaction with CSS Font Loading and Matching</h3>

	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 <a>font source</a> 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 <a method for=FontFace>load()</a> method
	of the corresponding <a interface>FontFace</a> object.

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

<h2 id='font-face-source'>
The <code>FontFaceSource</code> interface</h2>

	<pre class='idl'>
		interface <dfn interface>FontFaceSource</dfn> {
			readonly attribute <a interface>FontFaceSet</a> <dfn attribute for=FontFaceSource>fonts</dfn>;
		};

		Document implements <a interface>FontFaceSource</a>;
		WorkerGlobalScope implements <a interface>FontFaceSource</a>;
	</pre>

	Any document, workers, or other context which can use fonts in some manner must implement the <a interface>FontFaceSource</a> interface.
	The value of the context’s <a attribute>fonts</a> attribute is its <dfn>font source</dfn>,
	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 <a>font source</a> 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 <a interface>FontFace</a> objects within the <a>font source</a> are its <dfn>available font faces</dfn>.

<h3 id="document-font-face-set">
Interaction with CSS’s ''@font-face'' Rule</h3>

	The set entries for a document's <a>font source</a>
	must be initially populated with all the <a>CSS-connected</a> <a interface>FontFace</a> 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 <a>CSS-connected</a> <a interface>FontFace</a> objects
	must be added or removed from the document's <a>font source</a>,
	and maintain this ordering.

	All non-<a>CSS-connected</a> <a interface>FontFace</a> objects must be sorted after the <a>CSS-connected</a> ones,
	in insertion order.

	<p class='issue'>
		Can/should we include the local system fonts,
		or is that a fingerprinting problem?
		I think the set of local fonts is trivially discoverable anyway.
		Alternately, define a specialized variant of FontFace which represents all the local fonts collectively,
		and perhaps can be queried against?
		This would make FontFaceSet fully explain the current behavior of fonts.

<h2 id="font-load-event-examples">
API Examples</h2>

	<div class="example">
		To show content only after all font loads complete:

		<pre>
			document.FontFaceSet.ready().then(function() {
				var content = document.getElementById("content");
				content.style.visibility = "visible";
			});
		</pre>
	</div>

	<div class="example">
		Drawing text in a canvas with a downloadable font, explicitly
		initiating the font download and drawing upon completion:

		<pre>
			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);
		</pre>
	</div>

	<div class="example">
		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:

		<pre>
			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);
			}
		</pre>
	</div>

	<div class="example">
		The <a event>loadingdone</a> event only fires after all font related loads have completed
		<strong>and</strong> text has been laid out without causing additional font loads:

		<pre>
			@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");
			}

			body { font-family: latin-serif, jpn-mincho; }

			&lt;p>納豆はいかがでしょうか
		</pre>

		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 <a event>loadingdone</a>
		event fire.
	</div>


<h2 class="no-num" id="changes">Changes</h2>

<h3 class="no-num" id="changes-since-20130212">
Changes from the <a href="http://www.w3.org/TR/2013/WD-css3-fonts-20130212/">February 2013 CSS3 Fonts Working Draft</a></h3>

	Major changes include:

	<ul>
		<li>Add details of font load errors.

		<li>Switched to a promise-based design for load() and ready().

		<li>Renamed notifyWhenFontsReady() to ready().

		<li>Added a match() method, to directly expose the font matching algorithm.

		<li>Added a FontFace interface, to directly represent the individual font faces, and allow manual construction of font faces without having to indirect through a @font-face rule.

		<li>Dropped the per-font events, in favor of promise-based methods on FontFace.
	</ul>

<h2 class=no-num id=acknowledgments>
Acknowledgments</h2>

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