Except Chrome and Firefox both regularly have issues showing up on one platform and not the other, so "Chrome or Firefox testing environments" include the browser and all 3 primary platforms at the very least.
Oh look at that, one of those platforms being Microsoft Windows you've already paid for the license you need for testing FF and Chrome.
That may be the case, but it's quite possible you just didn't catch them. Most people really don't do a rigorous comparison and even if they do, it's so menial a task that the brain shuts down and things slip through the cracks.
I've dedicated the last 2.5 years of my life to auto-detecting differences between browsers and have tested thousands of sites across a variety of fields. I was kinda surprised at the number of rendering issues I've come across with Firefox and Chrome in various combinations of versions and OS. I've even seen some pretty substantial rendering differences between Win XP and Win 7 in Firefox.
Anyway, your experience could be totally different. I've just come across a lot devs that have adopted this hivemind mentality around Firefox & Chrome and as a result, they stop testing thoroughly with them. Without thorough testing you can't know if you have problems, so it's really hard to conclude anything because the key data are missing. There's a gap between perception and reality.
It's also possible that each issue, once figured out, became something I know how to avoid. There really just aren't that many things that break in a browser across operating systems.
Also, in my mind, there is a huge difference between "this font doesn't render pixel-perfect in this os/browser combo", and "the javascript broke". One causes much more customer support cost than the other.
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).
Oh look at that, one of those platforms being Microsoft Windows you've already paid for the license you need for testing FF and Chrome.