To play just use the keyboard (see keys assigned) or touch the screen as you would use a piano.
The HTMLlotron was built to pay tribute to the Mellotron. The Mellotron became famous when it was used as the signature sound for Strawberry Fields Forever. If you know the song (you probably do), you know the sound. Paul McCartney explains it better.
But the Mellotron was not an ordinary synth. In fact, it wasn't a synthesizer at all. I was a mechanical artifact that used tapes -yes, like cassette tapes- to play sounds recorder from real instruments. If you don't know about tapes, and cassettes, you are probably a centennial, but keep reading.
This weird machine ended being extraordinary famous because of The Beatles, and its sound remains one of the most memorables in contemporary music. In fact, Strawberry Fields used the "flute" sound, that is why this is the first set loaded in this virtual version.
In order to keep the spirit of the Mellotron, this online piano was build using sampled sounds that are fired when you hit a key. If you are curious, you can see this video to know in depth how it works.