Editorial: clarify the xmlns notes by merging them

Closes #11215.

Author: annevk

source

@@ -13669,13 +13669,9 @@ https://software.hixie.ch/utilities/js/live-dom-viewer/?%3C%21DOCTYPE%20HTML%3E%
 
   <p class="note">In HTML, the <code data-x="">xmlns</code> attribute has absolutely no effect. It
   is basically a talisman. It is allowed merely to make migration to and from XML mildly easier.
-  When parsed by an <span>HTML parser</span>, the attribute ends up in no namespace, not the "<code
-  data-x="">http://www.w3.org/2000/xmlns/</code>" namespace like namespace declaration attributes in
-  XML do.</p>
-
-  <p class="note">In XML, an <code data-x="">xmlns</code> attribute is part of the namespace
-  declaration mechanism, and an element cannot actually have an <code data-x="">xmlns</code>
-  attribute in no namespace specified.</p>
+  When parsed by an <span>HTML parser</span>, the attribute ends up in no namespace. In XML, the
+  attribute is part of the namespace declaration mechanism and always ends up in the "<code
+  data-x="">http://www.w3.org/2000/xmlns/</code>" namespace.</p>
 
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