application/xhtml+xml to a compliant user agent.
If you want to use XHTML markup but serve it as HTML, you're free to do so. It's quite acceptable, as long as it also works when served as an application of XML. If it must be served as HTML, then you are doing the wrong thing, and you cannot claim that there are any advantages to using XHTML (because you're not).
<abbr/HTML/ and <>CSS</> instead of the more bloated <abbr>HTML</abbr> and <abbr>CSS</abbr>, but that will never happen now.
XML is a subset of SGML with lots of restrictions which, in conjunction with the requirement for draconian error handling, makes it fairly trivial to write an XML parser. (Internal subsets are the only things that really complicate matters.)