HTML5 is still an evolving specification. The term is also commonly used to describe other web technologies that are not strictly part of HTML5 but which are being developed in parallel with it.
If, for the sake of argument, we consider the current HTML5 specification as 'HTML5', then there are no 100% compatible browsers in common circulation, not for mobile devices or desktop computers. However, most of HTML5 is just HTML4, which is well supported by all browsers.
WebKit-based browsers are presently some of the most advanced in implementing HTML5 and the surrounding technologies. This includes the iPhone versions of Safari.
I believe Sencha Touch 1 officially supports iOS 3 and later, which should cover the devices you've listed.