Piano plugged in, but Klavissimo isn't hearing anything? The most common causes rarely lie with the instrument itself — usually it's the cable, the browser, or a second program already holding the connection. Work through the list once.
Check the cable and power
Plug the USB cable directly into your computer or tablet — a different USB port or a new cable clears up most loose connections. Some E-pianos also need their own mains power to send data over USB at all; with no power at all, the port stays silent.
Free up the connection from another program
If a second program is already holding your piano open — a DAW, say, or another notation program running in the background — the browser can no longer reach it. Close the other program and reload Klavissimo.
Allow the browser permission
The first time round, the browser asks for permission to access MIDI devices — confirm it. If you accidentally chose "Block", Klavissimo shows "MIDI access denied" at the top; you can reset the permission afterwards in your browser's site settings.
Test the device at /midi-test
The "Instrument check" page shows live whether anything is arriving at all: under "1 · Keys" the keys you play light up, under "2 · Touch" you see your touch range, and under "3 · Pedal" it automatically detects whether your pedal reacts "✓ continuous" or is just a "Switch (0/127)".
Automatic on iPad
Using the Klavissimo app on iPad skips the whole browser question: it connects your piano through the native MIDI bridge, with no permission dialog at all.
Good to know:
Not every browser can do Web MIDI: Chrome and Edge work reliably, and Firefox has supported it as well since version 108. Only Safari can't connect a MIDI device at all — no update fixes that, only switching to Chrome or Edge.
Your data:
The raw-events log on /midi-test stays with you: "Copy" only puts the text on your clipboard, nothing is sent automatically. You decide yourself whether and how you pass it on.