Styles must be improved (full width for smartphones, ...)
PGN text is not selectable (understand why?!)
Change cursor when computer "think" (sablier)
Tooltip text should fade (even when mouse stay on it, especially for small screens)
Checkered stage 2: switch button at reserve position (or on top).
Mode expert: game.js, button on top (with online indicator)
Turn indicator on top too (black or white)