Keyboard focus must never get trapped

The check sorts the visible, keyboard-reachable elements into tab order: positive tabindex values first, ascending, then document order. It focuses the first that accepts focus, then presses the Tab key up to 200 times. If focus leaves the document, or the presses run out, nothing is reported.

A stall is focus staying on one element for 8 presses in a row. On an iframe, object or embed the check moves focus past it after 3 presses, so traps inside it go unreported. A loop is focus returning to an element already visited. A loop holding the first focused element and either the last tab stop or at least 80 percent of the tab stops is a complete pass; other loops are reported on their nearest common container.

The check then presses Escape and Tab. A trap that Escape releases, or any trap inside a dialog, role="dialog", role="alertdialog" or aria-modal="true" element, goes to needs review, which the scanner does not turn into findings today. Shift+Tab is not tested.