Most smartphones use modern browser engines that have implementing HTML 5 (or at least partially). Should I start using HTML 5 for my web application ? Where can I find a list of browser engines used by most popular devices (iPhone,Android,etc.) ? My application doesn't have to work on older desktop browsers.
Depending on how commercial/critical your web page is, the answer differs.
If it is CRITICAL that it works on ALL smart-phones, use HTML4, with a view to upgrading it to HTML5.
If it is preferable that it works on most smart-phones HTML5 will be fine. Even where it is not supported, the page will render - it just won't look like you intend. I'm using HTML5 with a couple of additions that help older browsers to render it correctly.
I did a browser test of HTML 5 (with the helpers for older browsers) with the following results:
http://www.stevefenton.co.uk/Content/Blog/Date/200907/Blog/HTML-5-Browser-Test/