Use .html extension instead of .xht extension inside html-templates/additions-to-the-css-user-agent-style-sheet
since they're not valid XHTML documents (e.g. missing html element) and need to be parsed as HTML documents.
Merge pull request #443 from rniwa/use-html-extension-for-tests-in-html-templates

Use .html extension instead of .xht extension inside html-templates/additions-to-the-css-user-agent-style-sheet