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
Parts of HTML 5 are supported by Safari, Firefox and Opera, but they are not necessarily incorporating the same parts.
It seems that Firefox is the most ahead, from my experience, but it will be years before the majority of browsers users use will support it.
So, until then we will need to continue trying to use it when we can, in browsers that support the new features, and having workarounds for users that haven't updated yet, or continue to use IE.
Use some services like http://caniuse.com/
For example for your question - http://caniuse.com/dataset
As you can see all modern browsers support it
Also you can use something like http://modernizr.com/ in your code (it's already included in http://html5boilerplate.com/)
PS: just notified that this question is too old, but it was linked to some other question i checked before
HTML5 isn't even close to being completely supported on any browser yet, and some browsers (notably the IE's) have no intention of supporting it at this time.