Cross-browser and layout testing is an integral part of testing web projects. The site will be displayed well and without bugs, but not in all browsers. Therefore, a significant amount of time, budget and resources is allocated for this testing.
Checking the site's display manually in various browsers is monotonous work that takes time. There are over 1500 different browsers at the moment. But, of course, most of them are unknown, that is why the testing is usually performed only in popular browsers and their versions (Google Chrome, Safari, Opera, Microsoft Edge, Firefox) and on such platforms as Windows, Linux, Mac.
The list of browsers and OS for cross-browser testing is approved before the testing. Mostly, there are enough free tools for cross-browser compatibility testing of layout for most projects without extra paid features.