Mobile browsers make a fuss about XHTML. In reality, mobile phones they support X-less HTML better, no matter what you are being told.
XHTML compatiblity is still needed for the search bots, if you wish to make your pages appear in the mobile search results. If you cannot provide XHTML compatible output, do not expect your pages appear in mobile search engines.
See Sniffing chapter for more information.
This need to be often changed to make your HTML XHTML compatible.
Do not use etc. HTML entity characters.
Do not use & in URL. Use & instead