Frame block
Overview
Section titled “Overview”This guide exercises the closed-dialect :::frame media frame end-to-end.

Invalid extra attrs stay inert rather than rendering a frame shell: W4B3_FRAME_INVALID_CANARY.
The exact-one media invariant also rejects prose children: W4B3_FRAME_STRUCTURE_INVALID_CANARY.
Shared prose after the frames remains on every surface: W4B3_FRAME_SHARED_PROSE.
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