Understanding SC 2.1.2 No Keyboard Trap
2.1.2 No Keyboard Trap: If keyboard focus can be moved to a component of the page using a keyboard interface, then focus can be moved away from that component using only a keyboard interface, and, if it requires more than unmodified arrow or tab keys or other standard exit methods, the user is advised of the method for moving focus away. (Level A)
Note: Since any content that does not meet this success criterion can interfere with a user’s ability to use the whole page, all content on the Web page (whether it is used to meet other success criteria or not) must meet this success criterion.
This success criterion requires that websites must not trap keyboard users in a particular portion of the page or on a form control. Keyboard users navigate the page using tab key. If the keyboard user is trapped in a particular location & is unable to move forward or backward using the navigational keys such as tab or shift+tab & user is forced to use a mouse to move keyboard focus then it fails this check point.