On the All Stars version, you can have Mario turn backwards when he touches the flagpole. How do you do that?

You must be using the Japanese version of the game. You can only turn around on the flag in some stages, like 8-1 and 8-2, and in some instances like in 4-1 you can do a setup (like hitting the mushroom block) that allows you to flip around on the flag pole. You don't even do anything different, Mario will automatically grab the right side of the flag. For Lost Levels though you can backwards jump onto any pole and you will flip to the right side.

Thanks!

Is html5 mario allowed?

To quote the rules;

NES Categories
Original NES, Famicom or FDS
Original NES or Famicom with Everdrive or Powerpak
Wii, Wii U, Switch and 3DS Virtual Console, GameBoy Advance
FCEUX, Nestopia, Bizhawk, OpenEMU
Frontends used in conjunction with these emulators that can alter the performance (ex. RetroArch) are not permitted
NES Classic Edition
Analogue NT Mini

SNES Categories

Original SNES or Super Famicom
Original SNES or Super Famicom with Super Everdrive or SD2SNES
Wii All-Stars Rerelease
Snes9x or Bizhawk
Frontends used in conjunction with these emulators that can alter the performance (ex. RetroArch) are not permitted
If you do not see a specific platform on this list, then it is not permitted.

So html5 mario is not allowed for run submissions.