[css-conditional-5] Add the named-feature() function.

This adds the named-feature() function but does not add any named
features.  (We did resolve on adding one, but it seems likely that it's
no longer useful at this point.  However, we likely will want others
soon.)

See resolutions in w3c#9875 and w3c#3559.

Author: dbaron

css-conditional-5/Overview.bs

@@ -117,12 +117,13 @@ Extensions to the ''@supports'' rule</h2>
 	<pre class="prod def" nohighlight>
 		<dfn>&lt;supports-feature></dfn> = <<supports-selector-fn>>
 		                   | <<supports-font-tech-fn>> | <<supports-font-format-fn>>
-		                   | <<supports-at-rule-fn>>
-						   | <<supports-decl>>
+		                   | <<supports-at-rule-fn>> | <<supports-named-feature-fn>>
+		                   | <<supports-decl>>
 		<dfn>&lt;supports-decl></dfn> = ( [ <<declaration>> | <<supports-condition-name>> ] )
 		<dfn>&lt;supports-font-tech-fn></dfn> = font-tech( <<font-tech>> )
 		<dfn>&lt;supports-font-format-fn></dfn> = font-format( <<font-format>> )
 		<dfn>&lt;supports-at-rule-fn></dfn> = at-rule( <<at-keyword-token>> )
+		<dfn>&lt;supports-named-feature-fn></dfn> = named-feature( <<ident>> )
 	</pre>
 
 	: <<supports-condition-name>>
@@ -150,6 +151,12 @@ Extensions to the ''@supports'' rule</h2>
 		<a href="#dfn-support-at-rule">supports the at-rule</a>
 		provided as an argument to the function.
 
+	: <<supports-named-feature-fn>>
+	::
+		The result is true if the UA
+		<a href="#dfn-support-named-feature">supports the named feature</a>
+		provided as an argument to the function.
+
 <h3 id="support-definition-ext">
 Extensions to the definition of support</h3>
 
@@ -186,6 +193,23 @@ At-rules</h4>
 	Note: Because ''@charset'' is not a valid [=at-rule=],
 	it is not considered to be supported under this definition.
 
+<h4 id="support-definition-named-features">
+Named features</h4>
+
+	A CSS processor is considered to
+	<dfn export for=CSS id="dfn-support-named-feature">support an named feature</dfn>
+	if it supports the named feature based on the feature definition
+	described in the following list:
+
+	[ No features are currently defined. ]
+
+	If the feature is not listed the processor does not support the named feature.
+
+	Note: The CSS Working Group intends to add features to this list rarely,
+	when there is real demand for feature testing something specific
+	and it would be unreasonable to add a more general feature testing mechanism
+	that would cover the use case.
+
 <h4 id="support-definition-supports-condition-name">
 Named conditions</h4>
 
@@ -2058,6 +2082,10 @@ Changes since the <a href="https://www.w3.org/TR/2024/WD-css-conditional-5-20241
 	<li>Made <<container-query>> optional
 		(<a href="https://github.com/w3c/csswg-drafts/issues/9192#issuecomment-1789850349">#9192</a>)
 	</li>
+	<li>Extended [=supports queries=] to express named features
+		via ''named-feature()''.
+		(<a href="https://github.com/w3c/csswg-drafts/issues/9875">#9875</a>)
+	</li>
 </ul>
 
 <h3 id="changes-20240723"  class="no-num">
@@ -2097,6 +2125,8 @@ Additions since Level 4</h3>
 			via ''font-tech()'' and ''font-format()''.
 		<li>Extended [=supports queries=] to express at-rule capabilities
 			via ''at-rule()''.
+		<li>Extended [=supports queries=] to express named features
+			via ''named-feature()''.
 		<li>Moved Container Queries from [[CSS-CONTAIN-3]] to this specification.
 			(See also the [[CSS-CONTAIN-3#changes]] for more information
 			on the evolution of this feature.)