Set createDocument() content type from namespace

Fixes #217.

dom.bs

@@ -449,6 +449,12 @@ The <dfn export>XML namespace</dfn> is
 The <dfn export>XMLNS namespace</dfn> is
 <code>http://www.w3.org/2000/xmlns/</code>.
 
+The <dfn export>SVG namespace</dfn> is
+<code>http://www.w3.org/2000/svg</code>.
+
+The <dfn export>MathML namespace</dfn> is
+<code>http://www.w3.org/1998/Math/MathML</code>.
+
 To <dfn export>validate</dfn> a <var>qualifiedName</var>, run these steps:
 
 <ol>
@@ -5089,6 +5095,23 @@ method, when invoked, must run these steps:
  <li><p><var>document</var>'s <a>origin</a> is the <a>origin</a> of the <a>context object</a>'s
  associated <a>document</a>. [[!HTML]]
 
+ <li><p><var>document</var>'s <a for=Document>content type</a> is determined by
+ <var>namespace</var>:
+
+ <dl class=switch>
+  <dt><a>HTML namespace</a>
+  <dd><code>application/xhtml+xml</code>
+
+  <dt><a>SVG namespace</a>
+  <dd><code>image/svg+xml</code>
+
+  <dt><a>MathML namespace</a>
+  <dd><code>application/mathml+xml</code>
+
+  <dt>Any other namespace
+  <dd><code>application/xml</code>
+ </dl>
+
  <li><p>Return <var>document</var>.
 </ol>