How to properly test sites in browsers and operating systems?

Question

Software testing

How to properly test sites in browsers and operating systems?
Now I'm sitting on windows 7 and only testing everything there.
I downloaded all the popular browsers that I have and support them, but do these sites need to be tested in windows XP, Vista, linux?
And how to support safari?
Please write down how any layout designer should test sites for cross-platform/cross-browser layout.

Answer 1

For testing, the operating system does not matter. Only the browser and its version.
PS: I was wrong. It is clear that the design will differ from the OS due to fonts, components, so I thought that only designers should bother the OS. Always dealt with the functionality of the page. But the fact that in IE with one version, but in different OS blocks can float , became news to me. This is more serious than just design. Now we need to fix not only the list of browsers, but also the OS for a specific version.