No Keyboard Trap

Understanding WCAG2.0

"Keyboard trap" is an implementation that makes it impossible to escape from keyboard operation when focus shifts to an element. When creating a modal window etc., you have to be able to do it not only with the mouse but also with the keyboard to close it.

Principle Operable User interface components and navigation must be operable.
Guideline Keyboard Accessible Make all functionality available from a keyboard.
Criterion No Keyboard Trap
(2.1.2 A)
If the keyboard interface can be used to move the keyboard focus to a component of the webpage, it is possible to remove focus from that component using only the keyboard interface. Furthermore, if you can not remove focus by using arrow keys without a modifier key, Tab key, or other standard method of removing focus, the user is notified of the method of removing focus.

URL of Techniques for WCAG 2.0