Nice game! I like the fact that it's made of "modules" which assemble together in a random fashion depending on the seed. The control feels nice and is quite addictive too.
I was wondering whether the jump key should be 'up' or maybe another key. Maybe in the keyboard is no big deal, but in the case the player's using a controller, binding the jump key to one of the action buttons might work better. In my case I love using the numeric keypad instead of the arrow keys for some strange reason, but it wasn't supported here.
I wonder whether it was intended, but the player loses air control upon hitting a wall or disengaging from a wall jump (i.e. sliding down the wall, once you're off it you won't be able to move either left or right mid-air, as would otherwise be possible for example after jumping off a ledge). This makes dealing with some sections trickier as you must carefully avoid hitting a wall if you don't want to fall straight down into the lava after that.
There's a pit I feel into that didn't kill me; the character went off-screen and I had to refresh the page to restart.
I didn't find any parts where the 'slide' mechanic (pressing 'down' before landing) was actually useful - I died most times I used it. It's a cool idea though; I was thinking there could be low corridors that require rolling to get through. Maybe this rolling could also be activated from a standstill by pressing the right keys.
I was thinking there could be alternate kinds of jump, just like in the game 'Hurdler' by KNPMASTER and MasterRaichu - long jump, short jump, and so on (maybe with different jump keys assigned?)
Overall, awesome game with great potential! Will make sure to keep tabs on this one.