This is probably the first TAS I made that I'm still proud of. It is very technical, especially in the secret levels, and everything is well optimized as far as I can tell.
Level-Specific Comments
1-3:
This level takes advantage of the low-gravity state Mario starts each level in. Mario's gravity resets each time he jumps, so as long as we do not jump, he will stay in this "floating" state. When this is combined with springs, we can bounce exceptionally high.
1-9:
The game would normally end here at the axe, but by opting to go through the pipe instead, we gain access to the secret levels.
1-A:
If the flagpole is grabbed normally, Mario will get hit by a firebar and be forced to continue the level until reaching another axe and defeating Bowser. However, by pushing Mario to the very right side of the screen, he will wrap around to the left after falling off of the flag pole and move on to 1-B
1-B:
A very obscure glitch is used here to raise a moving platform without Mario actually standing on it. I can't remember exactly how it works, but I believe it has to do with what frame the platform is loaded on after the previous platform drops off screen and despawns.
This hack can be downloaded here: https://www.romhacking.net/hacks/1583/