webgl for example, is implemented with angel on windows. angel is based on directx. there could be differences. I have a concrete example, but it is a long story. I'm too lazy to type. I might be able to find the link.

chrome video decoding is another example. on windows, you might be using hardware decoder. on mac, that could be a different story.

browser sandbox is also platform dependent.

depends on what you are testing. a thorough test might require you to test it on different versions of macs even. but that costs money and time.