Got that while playing a series of the zero-cost two-gold or two-damage cards. Nothing out of the ordinary.
Once I got that, refreshing the page and trying to restart the game had it stay on the screen that says “Running…” forever and never start. Eventually that stopped happening, but it happened several times in a row after getting the crash.
Just confirmed that I can reliably reproduce the same crash by playing the zero-cost two-gold and zero-cost two-damage cards in a loop. And afterwards, refreshing has the same result: stuck on the “Running…” screen.