What's missing in browsers for XHTML5? Don't they understand and validate the XML and allow using the HTML5 elements when some page is served with a application

What's missing in browsers for XHTML5? Don't they understand and validate the XML and allow using the HTML5 elements when some page is served with a application/xhtml+xml mimetype? Don't they, at worst, degrade to html5 parsing? (I only tried writing polyglot syntax, with only stuff allowed in both XHTML and HTML syntaxes) Or are you concerned with the unrecoverable mechanism?