There are currently four major versions of HTML and XHMTL:
XHTML 1 is basically the same as HTML 4, and does not add or change elements. The big change is that XHTML 1 uses a valid XML format, and XHTML 1.1 splits the specification in seperate modules.
Unfortunately, Internet Explorer does not support the correct XHTML 1 MIME Type: application/xhtml+xml, and so most authors write in XHTML, but specify the HTML 4 MIME-type (text/html). While the W3C validator accepts the results, it is formally invalid. Strangely, Internet Explorer does accept the application/xml MIME Type.
My recommended solution is to have the webbrowser serve
And put in each individual webpage
<meta http-equiv="Content-Type" content="application/xhtml+xml; charset=UTF-8" />