Useful but could use more...

The lists of JavaScript events and HTML attributes associated with each tag is really useful. It would be even better with a list of browser compatability and more detailed explanations (for instance, the JavaScript event docs dont describe how to extract event-related information from the DOM, so you might know that that an input has changed value, but not how to fetch the updated value).

But hey, its free and doesnt have ads!

Butterwaffle about HTML5 Reference Guide, v1.2