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 (via the FontFace
interface)
and loaded from script (via document.fonts).
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 Promise
s,
which are defined in ECMAScript 6.
MDN 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 CSSOMString = "normal";
style CSSOMString = "normal";
weight CSSOMString = "normal";
stretch CSSOMString = "U+0-10FFFF";
unicodeRange CSSOMString = "normal";
variant CSSOMString = "normal";
featureSettings CSSOMString = "normal";
variationSettings CSSOMString = "auto";
display CSSOMString = "normal";
ascentOverride CSSOMString = "normal";
descentOverride CSSOMString = "normal"; };
lineGapOverride enum {
FontFaceLoadStatus ,
"unloaded" ,
"loading" ,
"loaded" }; [
"error" Exposed =(Window ,Worker )]interface {
FontFace constructor (CSSOMString , (
family CSSOMString or BinaryData ),
source optional FontFaceDescriptors = {});
descriptors attribute CSSOMString family ;attribute CSSOMString style ;attribute CSSOMString weight ;attribute CSSOMString stretch ;attribute CSSOMString unicodeRange ;attribute CSSOMString variant ;attribute CSSOMString featureSettings ;attribute CSSOMString variationSettings ;attribute CSSOMString display ;attribute CSSOMString ascentOverride ;attribute CSSOMString descentOverride ;attribute CSSOMString lineGapOverride ;readonly attribute FontFaceLoadStatus status ;Promise <FontFace >load ();readonly attribute Promise <FontFace >loaded ; };
Clarify all mentions of "the document" to be clear about which document is being referenced, since objects can move between documents.
family
, of type CSSOMStringstyle
, of type CSSOMStringweight
, of type CSSOMStringstretch
, of type CSSOMStringunicodeRange
, of type CSSOMString-
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 astyle
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 corresponding @font-face descriptor. If it does not match the grammar, throw a
SyntaxError
; otherwise, set the attribute to the serialization of the parsed value. variant
, of type CSSOMStringfeatureSettings
, of type CSSOMStringvariationSettings
, of type CSSOMStringdisplay
, of type CSSOMStringascentOverride
, of type CSSOMStringdescentOverride
, of type CSSOMStringlineGapOverride
, of type CSSOMString-
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 corresponding @font-face descriptor. 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 onFontFace
, 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(family, source, descriptors)
method is called,
execute these steps:
-
Let font face be a fresh
FontFace
object. Set font face’sstatus
attribute to"unloaded"
, Set its internal[[FontStatusPromise]]
slot to a fresh pendingPromise
object.Parse the
family
argument, and the members of thedescriptors
argument, according to the grammars of the corresponding descriptors of the CSS @font-face rule. If thesource
argument is aCSSOMString
, parse it according to the grammar of the CSS src descriptor of the @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’sstatus
attribute to "error". Otherwise, set font face’s corresponding attributes to the serialization of the parsed values.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?
Return font face. If font face’s
status
is "error", terminate this algorithm; otherwise, complete the rest of these steps asynchronously. -
If the
source
argument was aCSSOMString
, set font face’s internal[[Urls]]
slot to the string.If the
source
argument was aBinaryData
, set font face’s internal[[Data]]
slot to the passed argument. -
If font face’s
[[Data]]
slot is notnull
, queue a task to run the following steps synchronously:-
Set font face’s
status
attribute to "loading". -
For each
FontFaceSet
font face is in:-
If the
FontFaceSet
’s[[LoadingFonts]]
list is empty, switch the FontFaceSet to loading. -
Append font face to the
FontFaceSet
’s[[LoadingFonts]]
list.
-
Asynchronously, 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:
-
If the load was successful, font face now represents the parsed font; fulfill font face’s
[[FontStatusPromise]]
with font face, and set itsstatus
attribute to "loaded".For each
FontFaceSet
font face is in:-
Add font face to the
FontFaceSet
’s[[LoadedFonts]]
list. -
Remove font face from the
FontFaceSet
’s[[LoadingFonts]]
list. If font was the last item in that list (and so the list is now empty), switch the FontFaceSet to loaded.
-
-
Otherwise, reject font face’s
[[FontStatusPromise]]
with a DOMException named "SyntaxError" and set font face’sstatus
attribute to "error".For each
FontFaceSet
font face is in:-
Add font face to the
FontFaceSet
’s[[FailedFonts]]
list. -
Remove font face from the
FontFaceSet
’s[[LoadingFonts]]
list. If font was the last item in that list (and so the list is now empty), switch the FontFaceSet to loaded.
-
-
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:
- Let font face be the
FontFace
object on which this method was called. - If font face’s
[[Urls]]
slot isnull
, or itsstatus
attribute is anything other than"unloaded"
, return font face’s[[FontStatusPromise]]
and abort these steps. - Otherwise,
set font face’s
status
attribute to "loading", return font face’s[[FontStatusPromise]]
, and continue executing the rest of this algorithm asynchronously. - Using the value of font face’s
[[Urls]]
slot, attempt to load a font as defined in [CSS-FONTS-3], as if it was the value of a @font-face rule’s src descriptor. -
When the load operation completes,
successfully or not,
queue a task to run the following steps synchronously:
-
If the attempt to load fails,
reject font face’s
[[FontStatusPromise]]
with a DOMException whose name is "NetworkError" and set font face’sstatus
attribute to "error".For each
FontFaceSet
font face is in:-
Add font face to the
FontFaceSet
’s[[FailedFonts]]
list. -
Remove font face from the
FontFaceSet
’s[[LoadingFonts]]
list. If font was the last item in that list (and so the list is now empty), switch the FontFaceSet to loaded.
-
-
Otherwise, font face now represents the loaded font;
fulfill font face’s
[[FontStatusPromise]]
with font face and set font face’sstatus
attribute to "loaded".For each
FontFaceSet
font face is in:-
Add font face to the
FontFaceSet
’s[[LoadedFonts]]
list. -
Remove font face from the
FontFaceSet
’s[[LoadingFonts]]
list. If font was the last item in that list (and so the list is now empty), switch the FontFaceSet to loaded.
-
-
If the attempt to load fails,
reject font face’s
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.
When a FontFace is transferred between documents, it’s no longer CSS-connected.
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.)
2.4. Discovery of information about a font
A FontFace
object includes a variety of read-only information about the contents of the font file.
[Exposed =(Window ,Worker )]interface { /* The CSSWG is still discussing what goes in here */ }; [
FontFaceFeatures Exposed =(Window ,Worker )]interface {
FontFaceVariationAxis readonly attribute DOMString ;
name readonly attribute DOMString ;
axisTag readonly attribute double ;
minimumValue readonly attribute double ;
maximumValue readonly attribute double ; }; [
defaultValue Exposed =(Window ,Worker )]interface {
FontFaceVariations readonly setlike <FontFaceVariationAxis >; }; [Exposed =(Window ,Worker )]interface {
FontFacePalette iterable <DOMString >;readonly attribute unsigned long ;
length getter DOMString (unsigned long );
index readonly attribute boolean ;
usableWithLightBackground readonly attribute boolean ; }; [
usableWithDarkBackground Exposed =(Window ,Worker )]interface {
FontFacePalettes iterable <FontFacePalette >;readonly attribute unsigned long ;
length getter FontFacePalette (unsigned long ); };
index partial interface FontFace {readonly attribute FontFaceFeatures ;
features readonly attribute FontFaceVariations ;
variations readonly attribute FontFacePalettes ; };
palettes
Note: This read-only data is intended to help authors know which values are accepted by font-feature-settings, font-variation-settings, and @font-palette-values.
3. The FontFaceSet
Interface
dictionary :
FontFaceSetLoadEventInit EventInit {sequence <FontFace >= []; }; [
fontfaces Exposed =(Window ,Worker )]interface :
FontFaceSetLoadEvent Event {(
constructor CSSOMString ,
type optional FontFaceSetLoadEventInit = {}); [
eventInitDict SameObject ]readonly attribute FrozenArray <FontFace >; };
fontfaces enum {
FontFaceSetLoadStatus ,
"loading" }; [
"loaded" Exposed =(Window ,Worker )]interface :
FontFaceSet EventTarget {constructor (sequence <FontFace >);
initialFaces setlike <FontFace >;FontFaceSet add (FontFace );
font boolean delete (FontFace );
font undefined clear (); // events for when loading state changesattribute EventHandler ;
onloading attribute EventHandler ;
onloadingdone attribute EventHandler ; // check and start loads if appropriate // and fulfill promise when all loads complete
onloadingerror Promise <sequence <FontFace >>load (CSSOMString font ,optional CSSOMString text = " "); // return whether all fonts in the fontlist are loaded // (does not initiate load if not available)boolean check (CSSOMString font ,optional CSSOMString text = " "); // async notification that font loading and layout operations are donereadonly attribute Promise <FontFaceSet >ready ; // loading state, "loading" while one or more fonts loading, "loaded" otherwisereadonly 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. FontFaceSet(initialFaces)
-
The
FontFaceSet
constructor, when called, must iterate itsinitialFaces
argument and add each value to its set entries. - iteration order
-
When iterated over, all CSS-connected
FontFace
objects must come first, in document order of their connected @font-face rules, followed by the non-CSS-connectedFontFace
objects, in insertion order. - set entries
-
If a
FontFaceSet
is a font source, its set entries are initialized as specified in § 4.2 Interaction with CSS’s @font-face Rule.Otherwise, its set entries are initially empty.
add(font)
-
When the
add()
method is called, execute the following steps:-
If font is already in the
FontFaceSet
’s set entries, skip to the last step of this algorithm immediately. -
If font is CSS-connected, throw an
InvalidModificationError
exception and exit this algorithm immediately. -
Add the font argument to the
FontFaceSet
’s set entries. -
If font’s
status
attribute is "loading":-
If the
FontFaceSet
’s[[LoadingFonts]]
list is empty, switch the FontFaceSet to loading. -
Append font to the
FontFaceSet
’s[[LoadingFonts]]
list.
-
-
Return the
FontFaceSet
.
-
delete(font)
-
When the
delete()
method is called, execute the following steps:-
If font is CSS-connected, return
and exit this algorithm immediately.false -
Let deleted be the result of removing font from the
FontFaceSet
’s set entries. -
If font is present in the
FontFaceSet
’s[[LoadedFonts]]
, or[[FailedFonts]]
lists, remove it. -
If font is present in the
FontFaceSet
’s[[LoadingFonts]]
list, remove it. If font was the last item in that list (and so the list is now empty), switch the FontFaceSet to loaded. -
Return deleted.
-
clear()
-
When the
clear()
method is called, execute the following steps:-
Remove all non-CSS-connected items from the
FontFaceSet
’s set entries, its[[LoadedFonts]]
list, and its[[FailedFonts]]
list. -
If the
FontFaceSet
’s[[LoadingFonts]]
list is non-empty, remove all items from it, then switch the FontFaceSet to loaded.
-
FontFaceSet
objects also have internal [[LoadingFonts]]
, [[LoadedFonts]]
,
and [[FailedFonts]]
slots,
all of which are initialized to empty lists,
and a [[ReadyPromise]]
slot,
which is initialized to a fresh pending Promise
.
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.
A FontFaceSet
is pending on the environment if any of the following are true:
-
the document is still loading
-
the document has pending stylesheet requests
-
the document has pending layout operations which might cause the user agent to request a font, or which depend on recently-loaded fonts
Note: The idea is that once a FontFaceSet
stops being pending on the environment,
as long as nothing further changes the document,
an author can depend on sizes/positions of things being "correct" when measured.
If the above conditions do not fully capture this guarantee,
they need to be amended to do so.
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:
- The
fontfaces
attribute is initialized to the result of filtering font faces to only containFontFace
objects contained in target.
When asked to switch the FontFaceSet to loading for a given FontFaceSet
,
the user agent must run the following steps:
- Let font face set be the given
FontFaceSet
. - Set the
status
attribute of font face set to "loading". - If font face set’s
[[ReadyPromise]]
slot currently holds a fulfilled promise, replace it with a fresh pending promise. - Queue a task to fire a font load event named
loading
at font face set.
When asked to switch the FontFaceSet to loaded for a given FontFaceSet
,
the user agent must run the following steps:
-
Let font face set be the given
FontFaceSet
. -
If font face set is pending on the environment, mark it as stuck on the environment, and exit this algorithm.
-
Set font face set’s
status
attribute to "loaded". -
Fulfill font face set’s
[[ReadyPromise]]
attribute’s value with font face set. -
Queue a task to perform the following steps synchronously:
-
Let loaded fonts be the (possibly empty) contents of font face set’s
[[LoadedFonts]]
slot. -
Let failed fonts be the (possibly empty) contents of font face set’s
[[FailedFonts]]
slot. -
Reset the
[[LoadedFonts]]
and[[FailedFonts]]
slots to empty lists. -
Fire a font load event named
loadingdone
at font face set with loaded fonts. -
If font face set’s failed fonts is non-empty, fire a font load event named
loadingerror
at font face set with failed fonts.
-
Whenever a FontFaceSet
goes from pending on the environment to not pending on the environment,
the user agent must run the following steps:
-
If the
FontFaceSet
is stuck on the environment and its[[LoadingFonts]]
list is empty, switch the FontFaceSet to loaded. -
If the
FontFaceSet
is stuck on the environment, unmark it as such.
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:
-
Parse font using the CSS value syntax of the font property.
If a syntax error occurs,
return a syntax error.
If the parsed value is a CSS-wide keyword, return a syntax error.
Absolutize all relative lengths against the initial values of the corresponding properties. (For example, a relative font weight like bolder is evaluated against the initial value normal.)
- If text was not explicitly provided, let it be a string containing a single space character (U+0020 SPACE).
- 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.
- Let available font faces be the available font faces within source. If the allow system fonts flag is specified, add all system fonts to available font faces.
- Let matched font faces initially be an empty list.
- 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. - If matched font faces is empty, set the found faces flag to false. Otherwise, set it to true.
-
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.
Note: Therefore, if text is the empty string, every font will be removed.
- Return matched font faces and the found faces flag.
3.2. The load()
method
The 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:
- Let font face set be the
FontFaceSet
object this method was called on. Let promise be a newly-created promise object. - Return promise. Complete the rest of these steps asynchronously.
- Find the matching font faces from font face set using the
font
andtext
arguments passed to the function, and let font face list be the return value (ignoring the found faces flag). If a syntax error was returned, reject promise with a SyntaxError exception and terminate these steps. -
Queue a task to run the following steps synchronously:
- For all of the font faces in the font face list,
call their
load()
method. - Resolve promise with the result of
waiting for all of the
[[FontStatusPromise]]
s of each font face in the font face list, in order.
- For all of the font faces in the font face list,
call their
3.3. The check()
method
The check()
method of FontFaceSet
will determine whether you can "safely"
render some provided text with a particular font list,
such that it won’t cause a "font swap" later.
If the given text/font combo will render without attempting to use any unloaded or currently-loading fonts,
this method will return true;
otherwise, it returns false.
-
If the specified fonts exist, but all possible faces are ruled out due to their unicode-range not covering the provided text, the method returns
true
, as the text will be rendered in the UA’s fallback font instead, and won’t trigger any font loads. -
Likewise, if none of the specified fonts exist (for example, names are mis-spelled), the method also returns
true
, because using this font list will not trigger any loads; instead, fallback will occur.
When the check
( font
, text
) method is called,
execute these steps:
- Let font face set be the
FontFaceSet
object this method was called on. - Find the matching font faces from font face set using the
font
andtext
arguments passed to the function, and including system fonts, and let font face list be the returned list of font faces, and found faces be the returned found faces flag. If a syntax error was returned, throw a SyntaxError exception and terminate these steps. - If font face list is empty,
or all fonts in the font face list either have a
status
attribute of "loaded" or are system fonts, returntrue
. Otherwise, returnfalse
.
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 [CSS-FONTS-3] 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.)
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
:
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
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
Mixin
interface mixin {
FontFaceSource readonly attribute FontFaceSet ; };
fonts Document includes FontFaceSource ;WorkerGlobalScope includes FontFaceSource ;
Any document, workers, or other context which can use fonts in some manner must include the FontFaceSource
mixin.
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 OffscreenCanvas
).
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 or shadow root CSS style sheets,
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.
Any manually-added FontFace
objects must be ordered after the CSS-connected ones.
When a FontFaceSet
object’s add()
method is called with a CSS-connected FontFace
object,
if the object is already in the set,
the operation must be a no-op;
otherwise, the operation must do nothing,
and throw an InvalidModificationError
.
When a FontFaceSet
object’s delete()
method is called with a CSS-connected FontFace
object,
the operation must be a no-op,
and return
.
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.
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
document. fonts. ready. then( function () { var content= document. getElementById( "content" ); content. style. visibility= "visible" ; });
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);
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); }
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:
- Added IDL for discovery of font information.
- Clarified that FontFaceSet.clear() does not clear CSS-connected items.
- Mentioed document.fonts in the introduction.
- Font loading applies to shadow roots as well as documents.
- No longer throw an error if none of the specified fonts exist, because this will not trigger any font loading.
- Better alignment with WebIDL.
- Switched to constructor() method syntax.
- Clarified behavior of the matching font faces algorithm if the string to match is empty.
- Converted FontFaceSource to a mixin.
- Legacy term CanvasProxy changed to OffscreenCanvas.
- Harmonized FontFace with @font-face, adding variationSettings and fontDisplay.
- Consistently use [Exposed] in the IDL.
- Prefer CSSOMString to DOMString
- Better introductory text for check()
- Clarified that layout operations which depend on recently-loaded fonts must be allowed to complete.
- Cover more edge cases when firing load events.
- Prefer async event queueing tasks over synchronous calls
- fonts.ready is a property, not a function.
- Differentiated between non-existing fonts, and fonts which exist but lack the required glyphs.
- Precisely listed order of steps for several methods
- Clarified handling of global keywords and relative values in the load() and check() functions.
- Parsing a src argument is the same as parsing a CSS @font-face src descriptor.
- Clarified that attempting to delete a CSS connected font has no effect, and returns false.
- Clarified that adding duplicate fonts has no effect.
- Clarified ordering of manually added FontFace object.
- Clarified that load events only include faces still present in the set.
- Added
variationSettings
anddisplay
, to sync with @font-face. - Switched
fontfaces
to be a FrozenArray, to match with proper IDL practice. - Fire loading events and handle promises when a loading font is added to a FontFaceSet.
- Corrected the async algorithms to use "queue a task" language, to ensure that side-effect timing is well-defined.
- Updated several references to latest versions.
- Corrections to the IDL.
- Assorted typos and grammatical errors corrected.
Acknowledgments
Several members of the Google Fonts team provided helpful feedback on font load events, as did Boris Zbarsky, Jonas Sicking and ms2ger.
Privacy Considerations
The FontFaceSet
object leaks information about the user’s installed fonts,
but in the exact same way as the existing @font-face rule;
no new information is leaked,
or in any appreciably easier manner.
Security Considerations
No security considerations have been raised against this specification.