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.
Deep look at the new generation interface