More fixes to element interface, and extends domintro

We should include in the eventual commit message:

In the process of upstreaming, changes were made to the "element interface" section to nail down underspecified edge cases, and fix https://www.w3.org/Bugs/Public/show_bug.cgi?id=27877.

source

@@ -9574,26 +9574,28 @@ interface <dfn>HTMLUnknownElement</dfn> : <span>HTMLElement</span> { };</pre>
   <span>HTML namespace</span> is determined as follows:</p>
 
   <ol>
-   <li><p>If <var>name</var> is <code>blink</code>, <code>bgsound</code>, <code>isindex</code>,
+   <li><p>If <var>name</var> is <code>bgsound</code>, <code>blink</code>, <code>isindex</code>,
    <code>multicol</code>, <code>nextid</code>, or <code>spacer</code>, return
    <code>HTMLUnknownElement</code>.</p></li>
    <!-- These have to be explicitly listed because technically we define these elements in HTML,
         albeit as obsolete. -->
 
-   <li><p>If this specification defines an interface appropriate for the element type corresponding
-   to the local name <var>name</var>, return that interface.</p></li>
+   <li><p>If <var>name</var> is <code>acronym</code>, <code>basefont</code>, <code>big</code>,
+   <code>center</code>, <code>listing</code>, <code>nobr</code>, <code>noembed</code>,
+   <code>noframes</code>, <code>plaintext</code>, <code>rb</code>, <code>rtc</code>,
+   <code>strike</code>, <code>tt</code>, or <code>xmp</code>, return
+   <code>HTMLElement</code>.</p></li>
 
-   <li><p>If this specifications defines the element type corresponding to the local name
-   <var>name</var>, but not an interface, return <code>HTMLElement</code>.</p></li>
-   <!-- Otherwise, e.g., <listing> would not be covered. -->
+   <li><p>Otherwise, if this specification defines an interface appropriate for the <span>element
+   type</span> corresponding to the local name <var>name</var>, return that interface.</p></li>
 
-   <li><p>If <span>other applicable specifications</span> define such an appropriate interface for
+   <li><p>If <span>other applicable specifications</span> define an appropriate interface for
    <var>name</var>, return the interface they define.</p></li>
 
    <li><p>If <var>name</var> is a <span>valid custom element name</span>, return
    <code>HTMLElement</code>.</p>
 
-   <li><p>Otherwise, return <code>HTMLUnknownElement</code>.</p></li>
+   <li><p>Return <code>HTMLUnknownElement</code>.</p></li>
   </ol>
 
   <p class="note">The use of <code>HTMLElement</code> instead of <code>HTMLUnknownElement</code> in
@@ -66874,12 +66876,12 @@ dictionary <dfn>ElementRegistrationOptions</dfn> {
 
    <dt><var>window</var> . <code data-x="dom-window-customElements">customElements</code> . <code
    data-x="dom-CustomElementsRegistry-define">define</code>(<var>name</var>, <var>constructor</var>,
-   { extends: <var>base</var> })</dt>
+   { extends: <var>baseLocalName</var> })</dt>
 
    <dd>Defines a new <span>custom element</span>, mapping the given name to the given constructor as
-   a <span>type extension</span> for the supplied <var>base</var> element. A
-   <code>NotSupportedError</code> will be thrown upon trying to extend a <span>custom element</span>
-   or an unknown element.</dd>
+   a <span>type extension</span> for the <span>element type</span> identified by the supplied
+   <var>baseLocalName</var>. A <code>NotSupportedError</code> will be thrown upon trying to extend a
+   <span>custom element</span> or an unknown element.</dd>
   </dl>
 
   <p><dfn>Element definition</dfn> is a process of adding a <span>custom element definition</span>