See example http://codepen.io/desandro/pen/CEfJF
The value of getComputedStyle( elem ).width is returned as 199px for an element that has CSS `width: 66.666%`, and it's parent element has `width: 300px`. Other browsers keep the fractional pixel value, i.e. 199.98px. I expected the value to either have the fractional pixel, or round up to the nearest integer, instead of rounding off any fractional pixel value.
I imagine this behavior might be by design, as it 199px is the rendered width of the element, and it prevents rounding errors to overshoot the true value. That said, I thought it was worth documenting.
I have found that using `calc()` values for `width` produces closer-to-expected values. So for that example, changing width to `calc( 100% * 2 /3 )` returns `200px`. See http://codepen.io/desandro/pen/CKwfd
getComputedStyle( box ).width now returns 199.984375px as expected. This has been fixed by one of the patches while transitioning to subpixel rendering.