I know testing websites on different browsers is important, but do browsers really render pages differently on different OSes?
For example, do I have to test Chrome for Mac, Linux and Windows to prove it works or is one fine?
For a long time, I only tested on Chrome/Firefox/Safari/IE/Opera on Windows, but about 2 years ago, I ran into a problem where the client was complaining about a screwed up layout on their Mac. I looked at the site in Adobe Browser Lab using OS X Safari and indeed it was rendering improperly compared to Safari for Windows.
Ever since then, I've stopped taking for granted that just because a browser renders the site properly on one OS that it'll be the same for other OSes.
This is true with certain JS/page loading behaviors as well, and doubly so for font rendering—especially as different OSes (especially Linux-based OSes) have different sets of fonts. There are just too many cross-platform factors to only test on a single OS.
Testing your site from a single OS in multiple browsers is sufficient enough to know if your code is cross browser compatible. I use Chrome as my default browser but also have Safari, FireFox, and IE for testing my websites from one computer.
Thank you for your interest in this question. Because it has attracted low-quality answers, posting an answer now requires 10 reputation on this site.
Would you like to answer one of these unanswered questions instead?