Of course, what are being discussed here are Graphical Browsers -- and the most popular ones at that. There are other User Agents (Browsers) used on the Internet -- Textual Browsers, Screen Readers, Speech Synthesizers, Braille Interpreters, etc. Here is an overview:
http://jp29.org/ua.htm
Following is a tabulation that depicts the number of extant Graphical Browsers in use:
http://www.w3.org/People/mimasa/test...-types/results
Following is a tabulation of the current market share for Graphical Browsers -- read the caveats:
http://www.upsdell.com/BrowserNews/stat.htm
BTW, Microsoft does not recognize XHTML documents served as content (MIME) type application/xhtml+xml. for MSIE 6.0/7.0. Their stated rationale is set forth at:
http://blogs.msdn.com/ie/archive/2005/09/15/467901.aspx
The foregoing decision by Microsoft has had a profound effect on the usage of XHTML as outlined at:
http://jp29.org/test.php