eXist 2.2 serializers now use the XQuery 3.0 rec. which breaks up the serialize instructions into individual declarations. So, according to the spec, you serialize as XHTML 5 like this:
declare namespace output = "http://www.w3.org/2010/xslt-xquery-serialization";
declare option output:method "xhtml";
declare option output:html-version "5.0";
declare option output:media-type "text/html";
Except, it looks like eXist may have a bug in there, because the above produces html without a doctype. To get around this use xhtml5 instead of just xhtml in the output:method declaration.
Also, we're using "text/html" for media-type instead of "application/xhtml+xml" because some stuff just doesn't work using xhtml+xml (Google Maps, specifically).
NOTE: Remember to change your page to use xquery 3.0