Someone I know came across this gem. I guess it's well publicised, but if you haven't come across it before, it could easily catch you out.
I have come across a menu system a while back that doesn't like IE7 because they had an IE-specific version of their code. Personally, I prefer to write for Firefox first (I find that Firefox has tended in the past to be slightly closer to the W3C standard), and then work on any code that doesn't work properly in IE, so that it displays correctly in both browsers WITHOUT having to have browser-detection. It can be hard, but it's definitely doable. I think it's much safer to stick as close as possible to the standard (but I do appreciate that this is a nice 'theory', and shouldn't be an enforced rule).
Correction: That page gives you the value that the current browser will give. Seems here, Firefox is wrong, as per the description of the function 'it should return 4 digits'. I had browsed to the page which said "this will give you 106", and figured it was correct, when actually, that was a browser quirk. Oh joy! (Thanks Alun)