[css-fonts-5] add incremental example, see #6063

Author: svgeesus

css-fonts-5/Overview.bs

@@ -29,6 +29,7 @@ spec:css-fonts-4; type:descriptor; text:font-language-override
 spec:css-fonts-4;  type:property; text:font-family
 spec:css-fonts-4;  type:property; text:font-style
 spec:css-fonts-4;  type:property; text:font-weight
+spec:css-fonts-4; type:descriptor; text:src
 </pre>
 
 
@@ -345,6 +346,71 @@ The <dfn id="at-font-face-rule">''@font-face''</dfn> rule</h3>
 	Multiple fonts can be used within an individual element
 	e.g. for characters not supported by the [=first available font=].
 
+<h3 id="src-desc">Font reference: the 'src' descriptor</h3>
+
+<h4 id="font-face-src-parsing">Parsing the 'src!!descriptor' descriptor</h4>
+
+	Note: this level adds the incremental font technology.
+
+
+	The 'src!!descriptor' descriptor value must be parsed
+	according to section [[css-syntax#parse-comma-separated-list-of-component-values]].
+	Then each component value is parsed according to this grammar:
+
+
+    <pre><<url>> [ format(<<font-format>> [supports <<font-technology>>#]?)]? | local(<<font-face-name>>)</pre>
+
+    <pre class="prod"><dfn id="font-format-values">&lt;font-format&gt;</dfn>
+		 = [<<string>> | woff | truetype | opentype
+		| woff2 | embedded-opentype | collection | svg]</pre>
+
+    <pre class="prod"><dfn id="font-technology-values">&lt;font-technology&gt;</dfn>
+		 = [features(<<font-feature-technology>>) | variations
+		| color(<<color-font-technology>>) | palettes
+		| incremental ]</pre>
+
+	<pre class="prod"><dfn id="font-feature-technology-values">&lt;font-feature-technology&gt;</dfn>
+		= [opentype | aat | graphite]</pre>
+
+    <pre class="prod"><dfn id="color-font-technology-values">&lt;color-font-technology&gt;</dfn>
+		= [COLRv0 | COLRv1 | SVG | sbix | CBDT ]</pre>
+
+    If a component value is parsed correctly
+	and is of a format and font technology that the UA supports,
+	add it to the list of supported sources.
+	If parsing a component value results in a parsing error
+	or its format or technology are unsupported,
+	do not add it to the list of supported sources.
+
+    If there are no supported entries at the end of this process,
+	the value for the 'src!!descriptor' descriptor is a parse error.
+
+    These parsing rules allow for graceful fallback of fonts
+    for user agents which don't support a particular font technology.
+
+	<div class="example" id="ex-incremental">
+		For example,
+		when incremental transfer is not supported,
+		a woff2 compressed version of the font
+		is supplied,
+		for optimal performance.
+		Then,
+		for incremental transfer using the
+		<a href="https://www.w3.org/TR/2020/NOTE-PFE-evaluation-20201015/#range-request">range-request method</a>,
+		the raw uncompressed OpenType font is provided
+		so that the client
+		can perform byte range requests.
+
+		<pre>
+		@font-face {
+			font-family: "MyIncrementallyLoadedWebFont";
+			src: url("FallbackURLForBrowsersWhichDontSupportIncrementalLoading.woff2") format("woff2");
+			src: url("MyIncrementallyLoadedWebFont.otf") format(opentype supports incremental);
+		}
+		</pre>
+	</div>
+
+
 <h3 id="font-size-desc">
 Font property descriptors: the 'font-size!!descriptor'
 </h3>