Usability testing is a method of testing the functionality of a website, app, or other digital product by observing real users as they attempt to complete tasks on it. In user testing they do different things to make sure that the website works without bugs and major design issues. Also they do cross browser testing to test how the page will interact with different browsers and make sure it is compatable with them. They do this testing by testing the page on mutiple stable browsers and accessibility testing such as testing to see if you can navigate the website with only your keyboard or testing on mobile devices.