Remove throw from set an attribute and move TT check down to replace …

…an attribute

dom.bs

@@ -6427,6 +6427,9 @@ boolean <var>validate</var> (default true), run these steps:
 <a>attribute</a> <var>oldAttr</var> with an <a>attribute</a> <var>newAttr</var>, run these steps:
 
 <ol>
+ <li><p><a>Validate and set attribute value</a> <var>newAttr</var>'s <a for="Attr">value</a> for
+ <var>newAttr</var> with <var>oldAttr</var>'s <a for=Attr>element</a>.
+
  <li><p><a for=list>Replace</a> <var>oldAttr</var> by <var>newAttr</var> in <var>oldAttr</var>'s
  <a for=Attr>element</a>'s <a for=Element>attribute list</a>.
 
@@ -6512,21 +6515,8 @@ string <var>namespace</var> (default null):</p>
  <var>attr</var>'s <a for=Attr>namespace</a>, <var>attr</var>'s <a for=Attr>local name</a>, and
  <var>element</var>.
 
- <li><p>If <var>oldAttr</var> is <var>attr</var>, return <var>attr</var>.
-
- <li>
-  <p>If <var>oldAttr</var> is non-null, then:</p>
-
-  <ul>
-   <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>.
-
-   <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>If <var>oldAttr</var> is non-null, then <a lt="replace an attribute">replace</a>
+ <var>oldAttr</var> with <var>attr</var>.
 
  <li><p>Otherwise, <a lt="append an attribute">append</a> <var>attr</var> to <var>element</var>.