[css-fonts-4] Migrate @font-face and 'font-family' descriptor from level 3

Author: litherum

css-fonts-4/Overview.bs

@@ -1050,6 +1050,120 @@ at least as dark as its predecessor. Only values between 1 - 999 are valid, and
 
 <h2 id="font-resources">Font Resources</h2>
 
+<h3 id="font-face-rule">The <dfn id="at-font-face-rule" style="font-weight: inherit; font-style: inherit"><code>@font-face</code></dfn> rule</h3>
+
+	The <code>@font-face</code> rule allows for linking to fonts that are
+	automatically fetched and activated when needed. This allows authors to select
+	a font that closely matches the design goals for a given page
+	rather than limiting the font choice to a set of fonts available
+	on a given platform. A set of font descriptors define the location of a font
+	resource, either locally or externally, along with the style characteristics of
+	an individual face. Multiple <code>@font-face</code> rules can be used to construct
+	font families with a variety of faces. Using CSS font matching rules, a
+	user agent can selectively download only those faces that are needed for
+	a given piece of text.
+
+	The <code>@font-face</code> rule consists of the <code>@font-face</code> at-keyword followed
+	by a block of descriptor declarations. In terms of the grammar, this
+	specification defines the following productions:
+
+	<pre><dfn>font_face_rule</dfn>
+	  : <i>FONT_FACE_SYM</i> <a href="https://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>* '{' <a href="https://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>* <i>descriptor_declaration</i>? [ ';' <a href="https://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>* <i>descriptor_declaration</i>? ]* '}' <a href="https://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>*
+	  ;
+
+	<dfn>descriptor_declaration</dfn>
+	  : <a href="https://www.w3.org/TR/CSS21/grammar.html#grammar"><i>property</i></a> ':' <a href="https://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>* <a href="https://www.w3.org/TR/CSS21/grammar.html#grammar"><i>expr</i></a>
+	  ;</pre>
+
+	The following new definitions are introduced:
+
+	<pre>-    -|\\0{0,4}2d(\r\n|[ \t\r\n\f])?
+
+	F    f|\\0{0,4}(46|66)(\r\n|[ \t\r\n\f])?</pre>
+
+	The following new token is introduced:
+
+	@{F}{O}{N}{T}{-}{F}{A}{C}{E} {return <dfn>FONT_FACE_SYM</dfn>;}
+
+	Each <code>@font-face</code>
+	rule specifies a value for every font descriptor, either implicitly or
+	explicitly. Those not given explicit values in the rule take the initial
+	value listed with each descriptor in this specification. These
+	descriptors apply solely within the context of the <code>@font-face</code> rule in
+	which they are defined, and do not apply to document language elements.
+	There is no notion of which elements the descriptors apply to or whether
+	the values are inherited by child elements.  When a given descriptor occurs
+	multiple times in a given <code>@font-face</code> rule, only the last
+	descriptor declaration is used and all prior declarations for that
+	descriptor are ignored.
+
+	<div class="example">
+
+	To use a downloadable font called Gentium:
+
+	<pre>
+	@font-face {
+	  font-family: Gentium;
+	  src: url(http://example.com/fonts/Gentium.woff);
+	}
+
+	p { font-family: Gentium, serif; }
+	</pre>
+
+	The user agent will download Gentium and use it when rendering text
+	within paragraph elements.  If for some reason the site serving the font
+	is unavailable, the default serif font will be used.
+	</div>
+
+	A given set of <code>@font-face</code> rules define a set of fonts
+	available for use within the documents that contain these rules.
+	When font matching is done, fonts defined using these rules
+	are considered before other available fonts on a system.
+
+	Downloaded fonts are only available to documents that reference them.
+	The process of activating these fonts must not make them available to
+	other applications or to documents that don't directly link to the same
+	font. User agent implementers might consider it convenient to use
+	downloaded fonts when rendering characters in other documents for which no
+	other available font exists as part of the <em>system font fallback</em> procedure.
+	However, this would cause a security leak since the contents of one page would be
+	able to affect other pages, something an attacker could use as an attack
+	vector.  These restrictions do not affect caching behavior, fonts are
+	cached the same way other web resources are cached.
+
+	This at-rule follows the forward-compatible parsing rules of CSS.
+	Like properties in a declaration block, declarations of any
+	descriptors that are not supported by the user agent must be ignored.
+	<code>@font-face</code> rules require a font-family and src
+	descriptor; if either of these are missing, the
+	<code>@font-face</code> rule is invalid and must be ignored
+	entirely.
+
+	In cases where user agents have limited platform resources or implement
+	the ability to disable downloadable font resources, <code>@font-face</code> rules must
+	simply be ignored; the behavior of individual descriptors as defined in this
+	specification should not be altered.
+
+<h3 id="font-family-desc">Font family: the <a href="#descdef-font-family">font-family</a> descriptor</h3>
+
+	<pre class='descdef mq'>
+	Name: font-family
+	Value: <<family-name>>
+	For: @font-face
+	Initial: N/A
+	</pre>
+
+	This descriptor defines the font family name that will be used in
+	all CSS font family name matching.  It is required for the <code>@font-face</code>
+	rule to be valid. It overrides the font family names contained in the
+	underlying font data. If the font family name is the same as a font
+	family available in a given user's environment, it effectively hides
+	the underlying font for documents that use the stylesheet.  This
+	permits a web author to freely choose font-family names without
+	worrying about conflicts with font family names present in a given
+	user's environment. Likewise, platform substitutions for a given font
+	family name must not be used.
+
 <h3 id="src-desc">Font reference: the 'src' descriptor</h3>
 
 	<pre class='descdef mq'>