Editorial: clarify namespace validate and extract

Fixes #1134.

Author: dlrobertson

dom.bs

@@ -210,8 +210,17 @@ run these steps:
 
  <li><p>Let <var>localName</var> be <var>qualifiedName</var>.
 
- <li><p>If <var>qualifiedName</var> contains a U+003A (:), then <a>strictly split</a> the string on
- it and set <var>prefix</var> to the part before and <var>localName</var> to the part after.
+ <li>
+  <p>If <var>qualifiedName</var> contains a U+003A (:), then:
+
+  <ol>
+   <li><p>Let <var>splitResult</var> be the result of running <a>strictly split</a> given
+   <var>qualifiedName</var> and U+003A (:).
+
+   <li><p>Set <var>prefix</var> to <var>splitResult</var>[0].
+
+   <li><p>Set <var>localName</var> to <var>splitResult</var>[1].
+  </ol>
 
  <li><p>If <var>prefix</var> is non-null and <var>namespace</var> is null, then <a>throw</a> a
  "{{NamespaceError!!exception}}" {{DOMException}}.