Summary

• capture contrast measurement PNGs with alpha preserved in both check and legacy validate
• skip a text candidate when its sampled backdrop remains transparent, because the downstream live-video background is unknown
• retain contrast checks for text over opaque cards/panels inside otherwise transparent compositions
• add browser-script regression coverage for transparent sampled pixels

Reproduction

A transparent 1080x340 composition with white overlay text fails every contrast sample at 1:1 because Chromium flattens the measurement screenshot onto white before the audit samples it. The actual WebM is composited over live video, so the claimed background and WCAG failure are invented.

Verification

• 56 focused CLI/browser-audit tests pass
• CLI typecheck passes
• formatting passes
• end-to-end transparent-root check --json changes from five hard contrast failures to a clean result with zero contrast candidates

Documentation note

The requested VP9 alpha-probe guidance already exists in docs/guides/rendering.mdx: check ALPHA_MODE=1 and decode with -c:v libvpx-vp9; plain native-decoder pix_fmt=yuv420p is expected.