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.
In plain language
If someone can reach a part of the page using only the keyboard, they must also be able to leave it using only the keyboard — never get stuck there.
Why it matters
Someone who can't use a mouse relies entirely on the keyboard to move around. If a component "traps" keyboard focus with no way out, that person is stuck and may have no way to use the rest of the page at all.
In practice
Fails this criterion
A custom pop-up widget captures all keyboard input and never releases it — pressing Tab, Escape, or any other key keeps focus locked inside, with no documented way out.
Meets this criterion
The same widget can be exited with a standard key like Escape or Tab, and if it needs a non-standard combination, that's clearly explained to the user beforehand.
Test it yourself
Navigate the whole page using only Tab, Shift+Tab, and arrow keys. If you ever get stuck inside a component with no way to move focus back out, this criterion is failing.