Throw InvalidStateError if the attribute being modified is modified b…

…y the default TT policy.

dom.bs

@@ -6353,16 +6353,21 @@ given a <var>document</var>, <var>localName</var>, <var>namespace</var>, and opt
 </ol>
 
 <p>To <dfn export id=concept-element-attributes-change lt="change an attribute">change</dfn> an
-<a>attribute</a> <var>attribute</var> to <var>value</var>, with string <var>sink</var>, run these steps:
+<a>attribute</a> <var>attribute</var> to <var>value</var>, with string <var>sink</var>, run these
+steps:
 
 <ol>
  <li><p>Let <var>oldValue</var> be <var>attribute</var>'s <a for=Attr>value</a>.</p></li>
 
  <li><p><a>Validate and set attribute value</a> <var>value</var> for <var>attribute</var>, with
  <var>attribute</var>'s <a for=Attr>element</a>, and <var>sink</var>.
 
- <li><p><a>Handle attribute changes</a> for <var>attribute</var> with <var>attribute</var>'s
- <a for=Attr>element</a>, <var>oldValue</var>, and <var>value</var>.
+ <li><p>If <var>attribute</var>'s <a for=Attr>element</a> <a lt="has an attribute">has</a>
+ an <a>attribute</a> <var>attribute</var>, then <a>handle attribute changes</a> for
+ <var>attribute</var> with <var>attribute</var>'s <a for=Attr>element</a>, <var>oldValue</var>, and
+ <var>value</var>.
+
+ <li><p>Otherwise, throw an "{{InvalidStateError!!exception}}" {{DOMException}}.
 </ol>
 
 <p>To <dfn export id=concept-element-attributes-append lt="append an attribute">append</dfn> an
@@ -6374,6 +6379,9 @@ string <var>sink</var> (default ""), run these steps:
  <var>attribute</var>'s <a for="Attr">value</a> for <var>attribute</var> with <var>element</var>,
  and <var>sink</var>.
 
+ <li><p>If <var>element</var> <a lt="has an attribute">has</a> an <a>attribute</a>
+ <var>attribute</var>, then throw an "{{InvalidStateError!!exception}}" {{DOMException}}.
+
  <li><p><a for=list>Append</a> <var>attribute</var> to <var>element</var>'s
  <a for=Element>attribute list</a>.
 
@@ -6497,7 +6505,11 @@ and a string <var>sink</var>:
    <li><p><a>Validate and set attribute value</a> <var>attr</var>'s <a for="Attr">value</a> for
    <var>attr</var> with <var>element</var>, and <var>sink</var>.
 
-   <li><p><a lt="replace an attribute">Replace</a> <var>oldAttr</var> with <var>attr</var>.
+   <li><p>If <var>element</var> <a lt="has an attribute">has</a> an <a>attribute</a>
+   <var>attr</var>, then <a lt="replace an attribute">replace</a> <var>oldAttr</var> with
+   <var>attr</var>.
+
+   <li><p>Otherwise, throw an "{{InvalidStateError!!exception}}" {{DOMException}}.
   </ul>
 
  <li><p>Otherwise, <a lt="append an attribute">append</a> <var>attr</var> to <var>element</var>