Update setAttributeNS to include TT check

Author: lukewarlow

dom.bs

@@ -7340,8 +7340,32 @@ method steps are:
  <li><p>Let <var>namespace</var>, <var>prefix</var>, and <var>localName</var> be the result of
  passing <var>namespace</var> and <var>qualifiedName</var> to <a>validate and extract</a>.
 
- <li><p><a>Set an attribute value</a> for <a>this</a> using <var>localName</var>, <var>value</var>,
- <var>prefix</var>, <var>namespace</var> and true.
+ <li><p>Let <var>attribute</var> be the result of
+ <a lt="get an attribute by namespace and local name">getting an attribute</a> given
+ <var>namespace</var>, <var>localName</var>, and <var>element</var>.
+
+ <li><p>If <var>attribute</var> is null, then set attribute to an <a>attribute</a> whose
+ <a for=Attr>namespace</a> is <var>namespace</var>, <a for=Attr>namespace prefix</a> is
+ <var>prefix</var>, <a for=Attr>local name</a> is <var>localName</var>, <a for=Attr>value</a> is
+ <var>value</var>, and <a for=Node>node document</a> is <var>element</var>'s
+ <a for=Node>node document</a>.
+
+ <li><p>Let <var>verifiedValue</var> be the result of
+ <a lt="verify attribute value">verifying an attribute value</a> given <var>value</var>,
+ <var>attribute</var>, and <var>element</var>.
+
+ <li><p>Set <var>attribute</var> to the result of
+ <a lt="get an attribute by namespace and local name">getting an attribute</a> given
+ <var>namespace</var>, <var>localName</var>, and <var>element</var>.
+
+ <li><p>If <var>attribute</var> is null, create an <a>attribute</a> whose <a for=Attr>namespace</a>
+ is <var>namespace</var>, <a for=Attr>namespace prefix</a> is <var>prefix</var>,
+ <a for=Attr>local name</a> is <var>localName</var>, <a for=Attr>value</a> is
+ <var>verifiedValue</var>, and <a for=Node>node document</a> is <var>element</var>'s
+ <a for=Node>node document</a>, then <a lt="append an attribute">append</a> this <a>attribute</a> to
+ <var>element</var>, and then return.
+
+ <li><p><a lt="change an attribute">Change</a> <var>attribute</var> to <var>verifiedValue</var>.
 </ol>
 
 <p>The