As noted in [1], much (most?) XHTML content on the web is actually
served with the "text/html" mime type, including the existing documens in the
dev server which declare themselves to be XHTML. However serving an
XHTML document with the correct "application/xhtml+xml" mime type alters
various browser behavior, so we need a page that tests this.

[1] https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/XHTML#xhtml_document