Ours was a one line focus fix, and it came from a player report rather than from us.
Browser games on itch run inside an iframe. If your key handlers are on window, keys only arrive while the frame has focus, so a player who clicks anywhere on the page around the game loses the keyboard. One player told us the controls just stopped working after a while. We checked our web games and 7 of 38 had it.
The fix is a pointer handler on the canvas that pulls focus back: window.focus() plus canvas.focus({ preventScroll: true }), with a tabindex on the canvas so it can take focus. It took minutes and it removed the most confusing thing a player could run into.
If you ship HTML5 builds, it's worth clicking outside your game once and then trying the keys.