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.
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