The hvc1 and 10-bit failures a few comments up aren't a FFmpeg-wasm fallback thing, they're a WebCodecs browser-gap. Firefox's HEVC path is partial and 10-bit paths are worse. Chrome mostly works and Firefox fails on the exact files iPhones and modern Androids record by default.

A "your browser can't decode this codec, try Chrome" nudge would probably spare people the bounce, especially on test imports from their phone.