[css-conditional-4] Clarify ns prefixes must be declared w3c#3220

Author: svgeesus

css-conditional-4/Overview.bs

@@ -28,6 +28,10 @@ WPT Display: open
 <pre class="link-defaults">
 	spec: css-conditional-3
 		type: dfn; text: conditional group rule
+	spec:css-namespaces-3;
+		type:dfn; text:namespace prefix
+	spec:css-namespaces-3;
+		type:dfn; text:CSS qualified name
 </pre>
 
 <h2 id="introduction">Introduction</h2>
@@ -55,7 +59,6 @@ Extensions to the ''@supports'' rule</h2>
 		provided as an argument to the function.
 
 <wpt>
-	at-supports-namespace-002.html
 	at-supports-selector-001.html
 	at-supports-selector-002.html
 	at-supports-selector-003.html
@@ -78,6 +81,42 @@ Extensions to the ''@supports'' rule</h2>
 	</pre>
 </div>
 
+Any namespace prefixes used in a [=conditional group rule=]
+must have been declared,
+otherwise they are invalid [[!css-conditional-3]].
+This includes namespace prefixes inside the selector function.
+
+<wpt>
+	at-supports-namespace-002.html
+</wpt>
+
+<div class="invalid example" id="ex-ns-selector-invalid">
+	This example tries to check that attribute selectors
+	with [=CSS qualified name=]s are supported,
+	but is invalid,
+	because the namespace prefix has not been declared.
+
+	<pre class="lang-css">
+		@supports selector(a[xlink|href]) {
+			// do something, but fail
+			}
+		}
+	</pre>
+</div>
+
+<div class="example" id="ex-ns-selector">
+	This example checks that attribute selectors
+	with [=CSS qualified name=]s are supported.
+
+	<pre class="lang-css">
+		@namespace x url(http://www.w3.org/1999/xlink);
+		@supports selector(a[x|href]) {
+			// do something
+			}
+		}
+	</pre>
+</div>
+
 <h3 id="support-definition-ext">
 Extensions to the definition of support</h3>
 
@@ -123,6 +162,9 @@ Changes since the <a href="https://www.w3.org/TR/2020/WD-css-conditional-4-20200
 
 	<ul>
 		<li>Added [[#privacy|Privacy]] and [[#security|Security]] sections.
+		<li>Added some examples</li>
+		<li>Clarified that the requirement to declare namespace prefixes
+			applies to selectors inside selector() (<a href="https://github.com/w3c/csswg-drafts/issues/3220">Issue 3220</a>)</li>
 	</ul>
 
 <h3 id="changes-from-L4" class="no-num">