User testing involves someone testing a website for usabilty. The tester is given various tasks to perform from the perspective of a first-time visitor. Another person observes the tester and takes notes about their experiences. Diffrent browser versions are tested in website user testing by going to the website in each browser version, and the tester documents any difficulties/bugs.
https://usabilitygeek.com/an-introduction-to-website-usability-testing/
https://www.softwaretestinghelp.com/how-is-cross-browser-testing-performed/