What's the official (or acceptable) sites to speedrun the web version?

world record is held in poki.com and stickman.pro

thanks!

I recommend using stickman.pro, although it's not required. I don't suggest using poki because it's an updated version so it's a lot worse.

what are the main differences between the updated version (poki) and the recommended version (stickman.pro)?

Poki is an updated version, while stickman.pro is not.

Anyone know how to reset back to lvl. 1 on stickman.pro?

Just use an incognito/private window and play it in there. I don't think there is any other way besides that.