Parser incorrectly assigns unknown_namespace to attributes in some WPT tests

[pshaughn]

html/syntax/serializing-html-fragments/serializing.html ends up putting unknown-namespace: on svg elements; since SVG support generally doesn't exist, the parser doing the wrong thing on just SVG elements isn't necessarily unexpected.

html/syntax/serializing-html-fragments/template.html does it on a div that is namespaced as XML and is a child of a template element. If I'm following it correctly, the right thing here is to use HTML semantics because template-ness should override XML-ness. Root cause could be the same as #24925 (but there the template is on the inside of the XML instead of the outside, so not necessarily the same).