The trickiest part of using HTML5 video is getting the video converted. Check out Easy HTML5 Video Converter if you need to make your videos available on multiple devices easily.
Devices running iOS 4.2 now support Web Sockets in Mobile Safari. This is great news for services that want to use Web Sockets and can’t rely on a Flash fallback.
iOS 4.2 also now supports TrueType Fonts and they look pretty snazzy.
The iOS devices have had great HTML5 and CSS3 support. LocalStorage works quite well, as do things like CSS3 transformations, pseudoclasses, and multi-column layouts. Offline support works great too, so it fou’yre looking for a neat way to build an offline mobile app for the iPad, you’ve got all the tools you need.