Use whenever writing, editing, or reviewing markup or UI code — HTML, JSX/TSX, Vue/Svelte/Angular templates, web components, email HTML, or CSS that affects text size, color, focus, motion, layout order, or hit area. Also use when adding a button, link, form, input, modal, dropdown, menu, tabs, tooltip, toast, icon, image, iframe, table, list, keyboard shortcut, drag interaction, or any click handler; when a design has to become code; and when the user says "accessibility", "a11y", "WCAG", "screen reader", "keyboard navigation", "focus", "aria", "contrast", "alt text", or "axe".