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

No.

The Wikipedia page "Comparison of layout engines (HTML 5)" does a good job of listing which engines have implemented which parts of HTML5.

There is currently a lot of red boxes on those tables, and that is based on the latest development version, not the version most users will be using.