What is the rule for parsing XML in a namespace inside HTML?

The subject says it.   If HTML has an XML 
namespace inside it, is the Draconian parse 
rule still in effect?

I can think of some wrinkles here, but they 
all lead to the conclusion that it is a bad 
idea to use XML inside HTML because the rules 
aren't the same.

And that could be a reason to deprecate HTML 
altogether.

len