У вашего броузера проблема в совместимости с HTML5
Somebody made a beta map for a possible new gamemode known as "Zombie Survival." The premise is similar to Mann vs. Machine, but the NPC zombies spawn constantly and in multiple areas.
It's about as buggy as you could expect, really. I've spoken to the mapmaker and he/she/it will fix them as soon as the SDK (Source Development Kit) is working again.