Is HTML 5 supported by all the main browsers?

Question

I am looking at the custom attributes feature of html 5 here at this link http://ejohn.org/blog/html-5-data-attributes/

This look like the perfect thing for when

Answer 1

Almost no web technology is completely supported by any browser; no bugs, quirks or issues.

HTML5 is designed for backwards compatibility, and it will hardly break your site (take for instance - non-supporting browsers show an ordinary text box, Opera lets you select an URL from history/bookmarks). I'd go by the approach: develop, try in the browsers you need to support - if it works, awesome. If not, don't use it. Just like with other specs.