Here are two approaches to a micro rhythm machine: Programmable Sequencer (specify the rhythm, then play it) and a Real-time Sequencer (push the keys at the right time to record a rhythm).
PROGRAMMABLE SEQUENCER
Left/Right: Move cursor
Up/Down: Specify which instrument will play
X: Play/Stop
Update 2017-04-25: Better feedback during editing.
REAL-TIME SEQUENCER
A really simple beat machine with real-time performance recording and playback on a two-bar loop. The metronome can't be switched off and the tempo can't be set. And the playback can't be paused/restarted. Also, only one drum can sound at a time. Still fun!!
The latency between image and audio is kinda hindering its feasibility, especially when run in a browser. Sequencers as music makers in PICO-8 are perhaps better approach, e.g. 4-beat by 4mat. Anyway, step editing would be cool though and I'll add it later (and some fancier graphics along with that).