This is an absolutely brilliant and mind-bending twist on the classic. I think anyone with a familiarity with Tetris will have a particularly difficult time (as I did) because you need to think about this game so differently in spite of its similar premise.
If I had a few minor suggestions it would be this: Up (or W) is typically quick-drop in Tetris, as I'm sure you're well aware. It would be nice to have this feature.
The game is also quite difficult. It just takes practice, of course, but I feel like the easy mode should be even easier-- so easy that somebody can spend the first few blocks just experimenting with the buttons and the mechanics.
As others have said, this needs a mobile version. EDIT: I somehow didn't see that the web version works on mobile. Problem is: it's played in landscape mode, which is uncomfortable. It also highlights the need (imo) for the game space to be taller.
PS: I accidentally bought the game on my other account (CatIllusion). I promise I'm not free-loading. :P Hahaha