FLV to MP4
Convert an old Flash Video (FLV, usually Sorenson Spark or H.263) to MP4. No browser decodes that codec through WebCodecs, so this runs through a 32MB ffmpeg engine you download once, after you consent, from this site.
FLV to MP4 turns an old Flash Video file, most often carrying Sorenson Spark or H.263, into a real MP4. WebCodecs has no decoder anywhere for either of those codecs, so the file instead goes through a 32MB ffmpeg engine, downloaded once after you tick a consent box. Leave that box unticked and the page refuses the file.
Questions
What codec makes FLV need the fallback?
Usually Sorenson Spark or H.263, neither of which any browser decodes through WebCodecs. mediabunny always refuses this codec family, so the page calls the 32MB ffmpeg.wasm engine directly rather than trying a browser-native attempt first.
Does consent apply the same way as the other ffmpeg pairs?
Yes. The fallback option is off by default and the same consent dialog avi-to-mp4 and wmv-to-mp4 use appears here too, so the download only starts after a real click.
Which engine handles this, and where can it run?
ffmpeg.wasm, downloaded once from this own site origin after consent, decodes and re-encodes, since mediabunny cannot open this codec at all. This is browser only; the command line and MCP refuse it, since there is no way to consent on those surfaces.
What size should I expect?
A real re-encoded MP4 with a genuine mdat box. The fixture test converts a one-second, 64 by 64 FLV clip to a similarly sized MP4, with the status text naming the ffmpeg engine that produced it.
What is the reverse page?
There is none. Nothing on this site writes FLV.