A lot of the same pieces falling after one another, sometimes 4 in a row. For deciding which pieces should appear, you should create an array with the 7 unique possibilites for pieces and then shuffle the array and go through all the 7 randomly-ordered pieces before doing the same process again. This guarantees all pieces appear consistently.
The game area is rendering off screen[0] for me in Firefox Nightly (63.0a1 (2018-08-27)) on macOS. Looks fine in Chrome and Safari.
Fun project. Watch out for TTC lawyers if it gets popular (: