[css-conditional-3] ns in conditional rules must be declared #3220

Author: svgeesus

css-conditional-3/Overview.bs

@@ -27,6 +27,8 @@ Default Highlight: css
 <pre class=link-defaults>
 spec:css-color-4; type:property; text:color
 spec:html; type:element; text:link
+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>
@@ -134,14 +136,36 @@ for some properties in [[CSS3-TRANSITIONS]] and
 <h2 id="contents-of">
 Contents of conditional group rules</h2>
 
-All [=conditional rules=] are defined to take a <<stylesheet>> in their block,
+All [=conditional group rules=] are defined to take a <<stylesheet>> in their block,
 which means they can accept any rule that is normally allowed at the top-level of a stylesheet,
 and not otherwise restricted.
 (For example, an ''@import'' rule must appear at the actual beginning of a stylesheet,
 and so is not valid inside of another rule.)
 
 Invalid or unknown rules inside the <<stylesheet>> must be considered invalid and ignored,
-but do not invalidate the [=conditional rule=].
+but do not invalidate the [=conditional group rule=].
+
+Any namespace prefixes used in a [=conditional group rule=]
+must have been declared,
+otherwise they are invalid.
+
+<div class="example" id=ex-declared-ns>For example, this rule
+  containing a [=CSS qualified name=] is valid,
+  because the [=namespace prefix=] has been bound to a namespace url:
+  <pre>
+    @namespace x url(http://www.w3.org/1999/xlink);
+    @supports (content: attr(x|href)) {
+      // do something }
+  </pre>
+</div>
+
+<div class="example">For example, to determine whether this rule is valid:
+  <pre>
+    @supports (content: attr(n|tooltip)) {
+      // do something }
+  </pre>
+The user agent will consult the namespace map to see whether a namespace url exists corresponding to the "n" prefix.
+</div>
 
 <h2 id="use">
 Placement of conditional group rules</h2>