[cssom] s/long/float/ for offsetLeft et al

Apologize if this has already been discussed, I'm not subscribed to  
www-style.

http://blogs.msdn.com/b/ie/archive/2012/02/17/sub-pixel-rendering-and-the-css-object-model.aspx

IE now apparently has a way to opt-in to making offsetLeft et al returning  
float values instead of long. Have MS done research that shows that  
returning floats by default Breaks The Web? If not, and it turns out that  
it isn't, should the spec be changed to return float?

-- 
Simon Pieters
Opera Software