Ask HN: Why WhastApp Web is currently only available on Google Chrome?

If I can ask more specifically, what are the features that is being used in WhatsApp Web such that it currently can be available only on Google Chrome? I've tested to trick WhatsApp Web through changing my User-Agent, and the Firefox & IE fails (errors in console). Hence I think there is some feature that is Chrome-specific only, but what is this feature? Can anyone point it out?

Thanks.