Using Firefox, the pixels of the highlighted squares showing your last move seem to be off. Running Firefox 82.0 (also happened on 81.x). I'm using the green board with the default pieces.
To recreate it, make a move. The highlighted squares showing your move may (it usually happens, but not always) be slightly off. If a light square is highlighted, a sliver of the light square may be showing on one side.
To recreate it, make a move. The highlighted squares showing your move may (it usually happens, but not always) be slightly off. If a light square is highlighted, a sliver of the light square may be showing on one side.