I am downloading loading webpages and munging the markup.  I strip
off the HTML namespace as part of the munging process but now I have
a requirement to reconstitute the HTML.

Can I leave the html unnamespaced or is it ok or  necessary to just
stick in an xmlns="http://www.w3.org/1999/xhtml" (or some other)
declaration

That appears to be an html question not an xslt one, so this is a _very_
marginal question for this list.

If you are generating HTML 4 or earlier any xmlns attribute is invalid.
If you are generating HTML5 or later
xmlns="http://www.w3.org/1999/xhtml" is valid but ignored.

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