# Header21 Implementation Packet

Implement `templates/blocks/hero-split-video-lightbox-left-copy-dual-cta.html` and `tests/components/header21-header.spec.cjs` after real RED.

Preserve the authenticated anatomy: one content-height split, mobile copy then media, desktop landscape lightbox left and copy right, exactly one h1, one description, two CTA destinations, one poster trigger, and one dialog/loader/iframe/video lifecycle. Provide no-JavaScript fallback, reduced motion, light/dark AA, 44px targets, focus trap/restore, Escape, close, backdrop, teardown, truthful image metadata, and overflow-safe rendering at 320/390/767/768/1280.

Use vanilla Light DOM and documented Ren10 APIs. Inline behavior must be `type="module"`, select the Header21 root once, and scope all descendant queries to it. Do not modify catalogs, inventory, shared APIs, tokens, components, or unrelated modules.