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Except for a few exceptions it seems like the solution is to write an emulator in JavaScript or Web Assembly that can load a flash file. Maybe the Internet Archive is working on something like that as they've already done for DOS, Commodore 64, Apple 2, etc. There are libraries out there that claim to run Flash (not sure to what level) so maybe compiling one of those into Web Assembly would be a start. Another is to petition Adobe to open source what they can.