DOCTYPEs; one more timeIf you’re sending your document as text/html over the wire you should use a DOCTYPE that triggers Standards Mode. I recommend <!doctype html>. Otherwise, if you’re sending your document with the media type being text/xml, application/xml or ending with +xml you can pick whatever DOCTYPE you like. I recommend not having one at all. Otherwise, fix your media type.
This implies that whether or not your document will validate for a given DOCTYPE doesn’t matter when picking one. This doesn’t mean font elements or target attributes are (always) ok to use. You should always strive for writing conforming documents. It means that if you have to use them you should not pick a DOCTYPE that triggers a less than standards parsing and rendering mode in the browser.