У вашего броузера проблема в совместимости с HTML5
Golly, that was close.
Still 100x easier than Undyne tho
This simulator is probably the most accurate one I've seen(the fact that I've only ever seen two notwithstanding), but of course it still has its flaws. For one, Sans doesn't speak, and he also doesn't attack on the menus on his second phase. Some of his attacks also last longer, while his Bone Stab attack is much easier here than in-game. The blue soul physics is also different and easier to grasp.
Nevertheless, I'd still recommend it as practice for the real thing. Just remember to bring snowman pieces, a sea tea, and the burnt pan.
Try out the simulator here: http://jcw87.github.io/c2-sans-fight/