I believe user experience is one of the requirements that is not explicitly mentioned in PRD but is as important as the user interface.
For say the requirement says the login page should be loaded, however, we rarely come across the requirement which says that the login page should load in 2-3 seconds.
Most of the time testers raise the issue based on their own experience that the experience is not good, which leads to discussion within the team and then the experience is modified or improved.