AFAIK the .fla format was never fully documented or reverse engineered by anyone (FFDEC has an exporter, but not importer), so this alone would be a bold claim.

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https://ruffle.rs/ is pretty solid

I'm talking about the .fla (XFL) format, not .swf (which is documented well - though that doesn't mean its exact behavior its understood well)

(note: I'm one of Ruffle's maintainers)

ruffle is a player for the output format (swf), .fla is the authoring format