I actually go well out of my way to avoid pixel-perfect stuff. The cases I mentioned all affect opacity or position by > 30px. There's actually a lot that can go wrong given the different windowing systems. I'd expect there to be more rendering bugs than JavaScript because of that. I can't speak to the functional side as well as I'd like, but I should ping someone at Sauce Labs to see what they've experienced.
Fair, but I was more alluding to the fact that shifted elements, while a nuisance, don't necessarily cause a site to stop functioning like JS can (assuming I'm working on a JS-heavy web app).
