clarify requirements on specs defining error handling of undeclared prefixes

Author: fantasai

css3-namespace/Overview.html

@@ -379,10 +379,11 @@ <h2 id=css-qnames><span class=secno>4. </span>CSS Qualified Names</h2>
    </div>
 
    <p>CSS qualified names can be used in (for example) selectors and property
-    values as described in other modules. Those modules should define the use
-    of a namespace prefix that has not been properly declared as a parsing
-    error that will cause the selector or declaration (etc.) to be considered
-    invalid and ignored.</p>
+    values as described in other modules. Those modules must define handling
+    of namespace prefixes that have not been properly declared. Such handling
+    should treat undeclared namespace prefixes as a parsing error that will
+    cause the selector or declaration (etc.) to be considered invalid and
+    ignored.</p>
   </div>
 
   <h2 class=no-num id=acknowledgments>Acknowledgments</h2>

css3-namespace/Overview.src.html

@@ -278,10 +278,11 @@ <h2 id="css-qnames">CSS Qualified Names</h2>
   </div>
 
   <p>CSS qualified names can be used in (for example) selectors and property
-    values as described in other modules. Those modules should define the use
-    of a namespace prefix that has not been properly declared as a parsing error
-    that will cause the selector or declaration (etc.) to be considered invalid
-    and ignored.</p>
+    values as described in other modules. Those modules must define handling
+    of namespace prefixes that have not been properly declared. Such handling
+    should treat undeclared namespace prefixes as a parsing error that will
+    cause the selector or declaration (etc.) to be considered invalid and
+    ignored.</p>
 </div>
 
 <h2 class="no-num" id="acknowledgments">Acknowledgments</h2>