<?xml version="1.0"?>
<!-- initially, the default namespace is "books" -->
<xml:namespace uri="urn:loc.gov:books">
<xml:namespace prefix="isbn" uri="'urn:ISBN:0-395-36341-6">
    <book>
    <title>Cheaper by the Dozen</title>
    <isbn:number>1568491379</isbn:number>
    <notes>
      <!-- drop the default into HTML for some commentary -->
      <xml:namespace uri='urn:w3-org-ns:HTML'>
      <p>
          This is a <i>funny</i> book!
      </p>
      </xml:namespace>
    </notes>
    </book>
</xml:namespace>
</xml:namespace>
If attribute-based namespace declaration is the only way to go, why not use
a simple word like 'namespace' instead of 'xmlns' so that its purpose is
clear to the reader?
If 'namespace' is too common, people can qualify it with 'xml' like this
'xml:namespace'?  Why not consider changing the name of the standard to
something shorter?
Saving of 4 characters does seem quite worth the use of obscure word like
'xmlns' for something as common as namespace.
Best wishes,
Don Park
CTO/Docuverse