Add tentative enforcement for toggleAttribute

dom.bs

@@ -6918,11 +6918,17 @@ method steps are:
  <p>If <var>attribute</var> is null, then:
 
  <ol>
- <li><p>If <var>force</var> is not given or is true, create an <a>attribute</a> whose
+ <li><p>Set <var>attribute</var> to an <a>attribute</a> whose
  <a for=Attr>local name</a> is <var>qualifiedName</var>, <a for=Attr>value</a> is the empty
- string, and <a for=Node>node document</a> is <a>this</a>'s <a for=Node>node document</a>, then
- <a lt="append an attribute">append</a> this <a>attribute</a> to <a>this</a>, and then return
- true.
+ string, and <a for=Node>node document</a> is <a>this</a>'s <a for=Node>node document</a>.
+
+ <li><p>Let <var>verifiedValue</var> be the result of calling <a>verify attribute value</a>
+ <var>attribute</var>'s <a for=Attr>value</a> for <var>attribute</var>, with <a>this</a>.
+
+ <li><p>Set <var>attribute</var>'s <a for=Attr>value</a> to <var>verifiedValue</var>.
+
+ <li><p>If <var>force</var> is not given or is true, then <a lt="append an attribute">append</a>
+ this <a>attribute</a> to <a>this</a>, and then return true.
 
  <li><p>Return false.
  </ol>