Abstract away comon logic in toggleAttribute() and setAttribute() to …

…be simpler

dom.bs

@@ -6189,6 +6189,24 @@ in an <a for="/">element</a> <var>element</var>, run these steps:
 
 <hr>
 
+To <dfn export id=concept-element-attributes-get-valid-by-name>get a valid attribute by name</dfn>
+given a <var>qualifiedName</var>, and <a for="/">element</a> <var>element</var>, run these steps:
+
+<ol>
+ <li><p>If <var>qualifiedName</var> does not match the <code><a type>Name</a></code> production in
+ XML, then <a>throw</a> an "{{InvalidCharacterError!!exception}}" {{DOMException}}.
+
+ <li><p>If <var>element</var> is in the <a>HTML namespace</a> and its <a for=Node>node document</a>
+ is an <a>HTML document</a>, then set <var>qualifiedName</var> to <var>qualifiedName</var> in
+ <a>ASCII lowercase</a>.
+
+ <!-- This is step 2 of "get an attribute by name", modified as appropriate -->
+ <li><p>Let <var>attribute</var> be the first <a>attribute</a> in <var>element</var>'s <a for=Element>attribute list</a>
+ whose <a for=Attr>qualified name</a> is <var>qualifiedName</var>, and null otherwise.
+
+ <li>Return <var>qualifiedName</var>, and <var>attribute</var>.
+</ol>
+
 To <dfn export id=concept-element-attributes-get-by-name>get an attribute by name</dfn> given a
 <var>qualifiedName</var> and <a for="/">element</a> <var>element</var>, run these steps:
 
@@ -6528,17 +6546,7 @@ method, when invoked, must these steps:
 method, when invoked, must run these steps:
 
 <ol>
- <li><p>If <var>qualifiedName</var> does not match the <code><a type>Name</a></code> production in
- XML, then <a>throw</a> an "{{InvalidCharacterError!!exception}}" {{DOMException}}.
-
- <li><p>If the <a>context object</a> is in the <a>HTML namespace</a> and its
- <a for=Node>node document</a> is an <a>HTML document</a>, then set <var>qualifiedName</var> to
- <var>qualifiedName</var> in <a>ASCII lowercase</a>.
-
- <li><p>Let <var>attribute</var> be the first <a>attribute</a> in <a>context object</a>'s
- <a for=Element>attribute list</a> whose <a for=Attr>qualified name</a> is <var>qualifiedName</var>,
- and null otherwise.
- <!-- This is step 2 of "get an attribute by name", modified as appropriate -->
+ <li><p>Let <var>qualifiedName</var> and <var>attribute</var> be the result of running <a>get a valid attribute by name</a> given the <var>qualifiedName</var> and <a>context object</a>
 
  <li><p>If <var>attribute</var> is null, create an <a>attribute</a> whose
  <a for="Attr">local name</a> is <var>qualifiedName</var>, <a for=Attr>value</a> is
@@ -6590,17 +6598,7 @@ when invoked, must run these steps:
 method, when invoked, must run these steps:
 
 <ol>
- <li><p>If <var>qualifiedName</var> does not match the <code><a type>Name</a></code> production in
- XML, then <a>throw</a> an "{{InvalidCharacterError!!exception}}" {{DOMException}}.
-
- <li><p>If the <a>context object</a> is in the <a>HTML namespace</a> and its
- <a for=Node>node document</a> is an <a>HTML document</a>, then set <var>qualifiedName</var> to
- <var>qualifiedName</var> in <a>ASCII lowercase</a>.
-
- <li><p>Let <var>attribute</var> be the first <a>attribute</a> in the <a>context object</a>'s
- <a for=Element>attribute list</a> whose <a for=Attr>qualified name</a> is <var>qualifiedName</var>,
- and null otherwise.
- <!-- This is step 2 of "get an attribute by name", modified as appropriate -->
+ <li><p>Let <var>qualifiedName</var> and <var>attribute</var> be the result of running <a>get a valid attribute by name</a> given the <var>qualifiedName</var> and <a>context object</a>
 
  <li>
   <p>If <var>attribute</var> is null, then: