HTML5 is now getting popular for developing mobile site. As far as I know, devices that work well with HTML5 is : Android, iOS, Symbian > 3rd edition, Blackberry with OS6, WebOS, and Windows Phone 7 (though it has very limited support). The other standard used is XHTML MP which work with low-end to high-end devices. But XHTML MP has very limited features compared to HTML5. I usually creating 2 mobile site, one for low-end with XHTML MP and one with HTML5 for mid-end to high-end.