Deal better with namespace undeclaration

lang/spec.html

@@ -1048,9 +1048,10 @@ <h5>XML namespaces</h5>
 <code>xml</code>. If there is already a namespace declaration in the [namespace
 attributes] that declares a prefix with that namespace name, then that prefix is
 used. Otherwise if there is a namespace declaration in the in-scope namespaces
-that declares a prefix with that namespace and it is not redeclared by the
-[namespace attributes], then that prefix is used. Otherwise generate a prefix
-and add an AII to the [namespace attributes] to declare it.</li>
+that declares a prefix with that namespace and it is not redeclared or
+undeclared by the [namespace attributes], then that prefix is used. Otherwise
+generate a prefix and add an AII to the [namespace attributes] to declare
+it.</li>
 <li>If the EII has no [namespace name] property, but a default namespace
 declaration is in scope, then an <code>xmlns=""</code> AII is added to the
 [namespace attributes] property to undeclare the default namespace.</li>
@@ -1066,8 +1067,8 @@ <h5>XML namespaces</h5>
 already declared as N; if there is one, set the [prefix] property of the EII to
 P;</li>
 <li>otherwise, if the [namespace attributes] property does not contain a default
-namespace declaration, generate a default namespace declaration for N and add it
-to the [namespace attributes] property;</li>
+namespace declaration or undeclaration, generate a default namespace declaration
+for N and add it to the [namespace attributes] property;</li>
 <li>otherwise, generate a new prefix P, set the [prefix] property of the EII to
 P, and add an AII to the [namespace attributes] to declare it.</li>
 </ol>