Think it's pretty obvious what the problem is here. Here's the suggested edit that the bug appears in.
Reproduced on both Firefox 38.0 and Chromium 43.0.2357.81 on Ubuntu 14.04.
After a bit of tinkering, the problem appears to be the
width: 900px CSS on the
table.chess-replayer-board element. Changing this to
width: 360px causes the board to display normally.