If you’ve ever tried to get precise em-based values of
letter-spacing to work in a WebKit browser, you’ve probably wondered why that isn’t really possible, while Firefox or Internet Explorer 10 for example do allow fine-grained control. Play around with this demo in various browsers and you’ll see what I mean.
The underlying reason is that WebKit currently uses integers to calculate pixel positions, while Firefox allows subpixel units to represent locations and sizes. This affects not only letter-spacing of course, but also things like rounding errors while zooming or applying transforms. There has been an ongoing effort to switch WebKit to use subpixel layout, and a few days ago Levi Weintraub and Emil A Eklund landed a patch that should put subpixel layout on the fast track. Look forward to more precision coming soon to WebKit-based browsers.
Here’s some technical background on this matter, and an article on subpixel rendering and the CSS Object Model from the IE team.