From what I've experienced, yes, it /is/ possible...

Buuuut it's really buggy for me, came here to ask if there's a way to fix it. I'm trying to use a Nintendo Switch Pro controller for Super Smash Flash 2 via the USB cord that it came with, however it keep randomly jittering up and down, thinking I'm inputting those actions when I'm not. (I don't even have any buttons bound.) I can't even bind anything because of this problem, as it keeps auto-inputting whatever 'BN0' is.

Please respond if you have an answer to my problem or if I'm just being dumb and this controller doesn't work with Smash Flash 2, thanks in advance.