This game is pretty good for a mario clone. I've played several browser platforms that don't perform super great in the past, but honestly the controls were pretty solid on this one. Still a little floaty, but honestly they worked well for me most of the time.
What I would definitely critique is the graphics and character design in general. I know this is a mario clone, but I think maybe that's worn on this games sleeve a little too apparently. Some unique enemies and characters would keep me from comparing it to mario so much.
There are also a few controls and UI gripes I have. Entering the level with the jump key wasn't super apparent so it took me a couple seconds to figure out that out. Also, once you've paused the game, you have to use the space bar to unpause it which was confusing.
Finally with the level coins, once you collect them, I think they stay collected, but every time you respawn they look the same, so I couldn't tell if they had been counted our not. Making them translucent or something would have fixed this. I had the same issue with the keyhole door where I unlocked one and then died, and wasn't sure if when I respawned the game still considered that I had unlocked it. Maybe you could change the graphic into a key in the lock once you get it so it's more clear.
Despite these gripes, I think the overall experience was really solid and I'd be down to try a sequel if one came out at some point.