This is a an emulated keyboard (a synthesizer!) that spans three musical octaves (C3-B5). Give it a shot, click any key with your mouse, or use the keys on your keyboard as indicated on the screen! (If you don't hear anything, try using Google Chrome or update your browser to the newest version. Mobile devices aren't all quite supported yet.)
All keyboards work by generating Waveform Audio file data dynamically, converting it into a base64-encoded dataURI, and subsequently playing it using the HTML5 audio element from within your web browser. Basically, we're using math to make sounds. Hooray. This is different than other HTML5 pianos in that it doesn't use pre-recorded audio files, and that it's a little bit more refined.