Ah, I remember doing this all too well. Or at least, with the very similar <meter> element anyway.

God it was a nightmare. There was no standardised selector for various parts of the bar, so I had to use a bunch of awkward -webkit and -moz prefixed ones that weren't consistent at all. The values for these were in at least one case contradictory, so the value for one part of the bar had to be assigned to one pseudo element in Chrome and the opposite one in Firefox, and getting the thing to work as expected in Safari was as awkward as you could imagine, despite all the documentation saying it'd use the same styles as Chrome.

They really need to standardise the selectors and styles for these things already...