Use HTMLPreElement for xmp and listing

Fixes #1015, and matches all non-Gecko engines. Before #1012 the standard was unclear on this; in #1012 the standard sided with Gecko, but per #1015 it is better to side with the majority.

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@@ -9617,10 +9617,12 @@ interface <dfn>HTMLUnknownElement</dfn> : <span>HTMLElement</span> { };</pre>
    <code>HTMLUnknownElement</code>.</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>, then return
-   <code>HTMLElement</code>.</p></li>
+   <code>center</code>, <code>nobr</code>, <code>noembed</code>, <code>noframes</code>,
+   <code>plaintext</code>, <code>rb</code>, <code>rtc</code>, <code>strike</code>, or
+   <code>tt</code>, then return <code>HTMLElement</code>.</p></li>
+
+   <li><p>If <var>name</var> is <code>listing</code> or <code>xmp</code>, then return
+   <code>HTMLPreElement</code>.</p></li>
 
    <li><p>Otherwise, if this specification defines an interface appropriate for the <span>element
    type</span> corresponding to the local name <var>name</var>, then return that interface.</p></li>