Salamander Grand Piano
The piano sound is the Salamander Grand Piano V3 by Alexander Holm, recorded from a Yamaha C5 and released under the Creative Commons Attribution 3.0 Unported license (creativecommons.org/licenses/by/3.0). Source: archive.org/details/SalamanderGrandPianoV3.
This site uses one velocity layer of the compressed MP3 versions published by the Tone.js Piano project (github.com/tambien/Piano). The files are served as published; during playback the browser trims the silence at the start of each recording, shortens its tail with a fade and shifts its pitch by up to one semitone to reach the notes in between. Those changes happen only on your device.
Keyboard diagrams
The labeled keyboard diagrams are drawn by the site's own code.