I think it depends on your locale, but it’s probably safe to assume emulators themselves are perfectly legal.
The only thing about emulation that’s illegal (to my knowledge) is the distribution of copyrighted software (i.e. “ROMs.”) Although, use of certain information in making the emulator (such as violation of NDA’s, etc.) might get an emulator author in trouble…
But, as long as you only play public domain ROM images, (and, of course, those created by yourself) you should be fine.
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