There are a number of in-browser beat making tools, many of them replicating classic hardware. Here we have the seminal combination of the TR-808 and TB-303 available to play and program alongside each other. It's a collaboration with the Design Museum which hosted an Electronic exhibition late last year - and Yuri Suzuki.
Both are simple to sequence, with the TB-303 being more notably accessible than the original machine, with a step sequencer and random feature - and the 808 offering an AI Variation mode. Alternative options for browser beat making come from the acid machine and the io808 while the HTML909 appears to have disappeared.