Clarify handling of foreign elements and attributes (#213)

imsc1/spec/ttml-ww-profiles.html

@@ -347,19 +347,10 @@ <h3>Foreign Element and Attributes</h3>
       <p>A <a>transformation processor</a> SHOULD preserve such elements or attributes whenever possible.</p>
 
       <div class="note">
-        <a data-lt="Document Instance">Document Instances</a> remain subject to the content specification for the <code>tt</code>
-        root document element and its descendants, as specified in [[!TTML1]]. In particular, the [[!TTML1]] content specification
-        specifies that elements and attributes not in any TT namespace, i.e. in foreign namespaces, are permitted as follows:
-
-        <ul>
-          <li>the <code>metadata</code> element is the only element whose child elements can belong to foreign namespaces, and the
-          <code>metadata</code> element is permitted on all elements defined by [[!TTML1]] with the exception of the following:
-          <code>ttp:feature</code>, <code>ttp:extension</code>, <code>tt</code>, <code>style</code>, <code>ttm:title</code>,
-          <code>ttm:desc</code>, <code>ttm:copyright</code>, <code>ttm:agent</code>, <code>ttm:name</code>, and
-          <code>ttm:actor</code>; and</li>
-
-          <li>foreign namespace attributes are permitted on all elements defined by [[!TTML1]].</li>
-        </ul>
+        <p><a data-lt="Document Instance">Document Instances</a> remain subject to the content conformance requirements specified
+        at Section 3.1 of [[!TTML1]]. In particular, a <a>Document Instance</a> can contain elements and attributes not in any TT
+        namespace, i.e. in foreign namespaces, since such elements and attributes are pruned by the algorithm at Section 4 of
+        [[!TTML1]] prior to evaluating content conformance.</p>
       </div>
 
       <div class="note">