Connect a digital piano or MIDI keyboard to your computer and practice mode becomes available. The music stops just before each next note and moves on when you press the right keys, so you can find the notes at your own pace - which helps most while you are still learning them.
What you need, and which browsers work
You need a digital piano or keyboard with MIDI output and a USB cable to connect it. Most digital pianos are recognised as soon as you connect the USB port on the back (often labelled "TO HOST") to the computer.
The browser needs Web MIDI, which Chrome and Edge on a computer provide. Safari, and browsers on iPhone and iPad, do not, so the practice mode button stays disabled (hovering over it shows "Your browser does not support Web MIDI API"). The first time, the browser may ask for permission to use MIDI devices; allow it.
Starting practice mode
Press the Practice Mode button at the top right of the player and a small panel opens. Press Start Practice Mode: playback stops and the piece starts again from the beginning. That is also when the browser first reaches for MIDI devices, so until you have pressed it once after opening the page, the panel will not say "MIDI keyboard detected" even with the keyboard plugged in. To practise from somewhere else, move the seek bar.
Press the same button again (now Watch Mode) to return to normal playback, carrying on from where practice mode had reached. In practice mode the falling notes are always shown, even for pieces with sheet music.
How the waiting works
The player treats notes that start within 0.05 seconds of each other as one chord. The notes to play next glow; press every note of that chord and the music runs on at its written tempo. When it reaches the next chord it carries straight on if those keys are already down, and stops to wait if any are missing. Where the hands start a note together, both have to be pressed.
Wrong keys still sound but do not hold you back; the music moves on as soon as the needed notes are all down. A key held over from the previous chord counts if the next chord needs it; keys only have to be struck afresh right after starting practice mode or moving the seek bar. Reach the end and "Performance Complete!" appears.
What practice mode does not check
Practice mode only checks whether the right keys went down. Rhythm, timing, note lengths and dynamics are not judged. Because the music waits for you, the playback speed and loop settings do not apply, and there is no setting to check one hand only: it always asks for every note in the file.
In other words, practice mode is for learning where the notes are and in what order. Once you know them, go back to watch mode, slow it down and use the loop to play along in rhythm. Hands-separate work also suits watch mode, following one colour.
Sorting out the sound
In practice mode the player sounds the keys you press on its own piano. If that doubles up with your digital piano's own sound, turn one of them down; the player's volume slider and mute apply to the notes you play as well.
The damper pedal on your keyboard is not used in practice mode, so the player's sound stops when you release a key. If you want to hear the pedal, mute the player and listen to your digital piano itself.