YouTube offers no 4K in Safari: no hardware VP9 / AV1 decoder #8

Safari's YouTube quality menu stops below 2160p. YouTube serves 4K only as VP9 or AV1, and Safari offers those only with a hardware decoder. The driver registers hardware decoders for H.264 and HEVC only (VTIsHardwareDecodeSupported is false for AV1 and VP9; see #3, #4). Blackwell's NVDEC can decode both, so this needs upstream decoder work.

Tested on RTX 5070 Laptop, macOS 26.7.1, stock NullMoth 2.3.7 (guided check by Bash). Evidence: project-legion evidence/native/featuretest-2026-10-18/A/vid.g.youtube.png, vid.vt.txt.