No Keyboard Trap
Keyboard focus must be able to move away from any component using only the keyboard — there must be no "trap" where Tab stops moving focus.
What it requires
WCAG 2.0 SC 2.1.2 requires that any component receiving keyboard focus also has a keyboard mechanism for focus to leave it. The most common offenders: modal dialogs that trap focus permanently, custom date pickers with no escape, and embedded videos that capture focus and don't release.
The fix pattern for modals: trap focus inside while open, return focus to the triggering button on close, and listen for the Escape key to close.
Common Shopify failure
Cart drawer that traps Tab focus indefinitely once opened — Escape does nothing, clicking the close X is the only way out (impossible for keyboard-only users). Newsletter modal with no close button reachable by Tab.
How to fix it
AccessComply rewrites cart-drawer and newsletter-modal Liquid templates to handle Escape key, expose a focusable close button as the first or last tab stop, and return focus to the original trigger when closed.
Primary source: W3C — WCAG 2.0 Understanding 2.1.2