User focus is not accidentally trapped in a region
If you create a pop up modal, make sure that the Close button has focus state Focused (Keyboard) and can be closed using the ESC key.
This might also be a problem with some plug-ins. For example, if you have a shopping cart plug-in, it must be possible to return to the main site using the keyboard.
Google Lighthouse says
Keyboard focus should never be locked or trapped at one particular page element. Users should be able to navigate to and from all page elements using only the keyboard.