[css-conditional-5] Spec at-rule() per CSSWG resolutions. #2463, #6966, #11116, #11117 (#12945)

* [css-conditional-5] Spec at-rule() per CSSWG resolutions. #2463, #6966, #11116, #11117

* Update css-conditional-5/Overview.bs

* Add to "Additions since Level 4"

---------

Co-authored-by: Tab Atkins Jr. <jackalmage@gmail.com>

Author: kbabbitt

css-conditional-5/Overview.bs

@@ -114,18 +114,15 @@ Extensions to the ''@supports'' rule</h2>
 	This level of the specification extends the <<supports-feature>> syntax as follows:
 
 	<pre class="prod def" nohighlight>
-		<dfn>&lt;supports-feature></dfn> = <<supports-selector-fn>> | <<supports-font-tech-fn>>
-							| <<supports-font-format-fn>> | <<supports-decl>>
+		<dfn>&lt;supports-feature></dfn> = <<supports-selector-fn>>
+		                   | <<supports-font-tech-fn>> | <<supports-font-format-fn>>
+		                   | <<supports-at-rule-fn>>
+		                   | <<supports-decl>>
 		<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>> )
 	</pre>
 
-	<wpt>
-		at-supports-font-format-001.html
-		at-supports-font-tech-001.html
-		js/CSS-supports-L5.html
-	</wpt>
-
 	: <<supports-font-tech-fn>>
 	::
 		The result is true if the UA
@@ -138,9 +135,24 @@ Extensions to the ''@supports'' rule</h2>
 		<a href="#dfn-support-font-format">supports the font format</a>
 		provided as an argument to the function.
 
+	: <<supports-at-rule-fn>>
+	::
+		The result is true if the UA
+		<a href="#dfn-support-at-rule">supports the at-rule</a>
+		provided as an argument to the function.
+
 <h3 id="support-definition-ext">
 Extensions to the definition of support</h3>
 
+<h4 id="support-definition-ext-fonts">
+Font techs and formats</h4>
+
+	<wpt>
+		at-supports-font-format-001.html
+		at-supports-font-tech-001.html
+		js/CSS-supports-L5.html
+	</wpt>
+
 	A CSS processor is considered to
 	<dfn export for=CSS id="dfn-support-font-tech">support a font tech</dfn>
 	when it is capable of utilizing the specified [[css-fonts-4#font-tech-definitions]]
@@ -152,6 +164,21 @@ Extensions to the definition of support</h3>
 	in layout and rendering,
 	and this format is not specified as a <<string>>.
 
+<h4 id="support-definition-at-rules">
+At-rules</h4>
+
+	<wpt>
+		js/supports-at-rule.html
+	</wpt>
+
+	A CSS processor is considered to
+	<dfn export for=CSS id="dfn-support-at-rule">support an at-rule</dfn>
+	if it would accept an [=at-rule=] beginning with the specified at-keyword
+	within any context.
+
+	Note: Because ''@charset'' is not a valid [=at-rule=],
+	it is not considered to be supported under this definition.
+
 <h2 id="when-rule">
 Generalized Conditional Rules: the ''@when'' rule</h2>
 
@@ -1812,6 +1839,14 @@ Changes since the <a href="https://www.w3.org/TR/2024/WD-css-conditional-5-20241
 		Dimensional query containers no longer apply layout containment
 		(<a href="https://github.com/w3c/csswg-drafts/pull/10544">#10544</a>)
 	</li>
+	<li>
+		Extended [=supports queries=] to express [=at-rule=] capabilities
+		via ''at-rule()''.
+		(<a href="https://github.com/w3c/csswg-drafts/issues/2463">#2463</a>,
+		<a href="https://github.com/w3c/csswg-drafts/issues/6966">#6966</a>,
+		<a href="https://github.com/w3c/csswg-drafts/issues/11116">#11116</a>,
+		<a href="https://github.com/w3c/csswg-drafts/issues/11117">#11117</a>)
+	</li>
 </ul>
 
 <h3 id="changes-20240723"  class="no-num">
@@ -1849,6 +1884,8 @@ Additions since Level 4</h3>
 		<li>Added ''@when'' and ''@else''.
 		<li>Extended [=supports queries=] to express font capabilities
 			via ''font-tech()'' and ''font-format()''.
+		<li>Extended [=supports queries=] to express at-rule capabilities
+			via ''at-rule()''.
 		<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.)