So in reality you don't develop against standards (alas), you develop against their implementations, and having multiple engines helps in no way.
But the result is that it's very rare to find cases that are really developed "to standard".
Even worse, though, on mobile right now people aren't even trying to develop to standards. UA sniffing and locking out of non-WebKit UAs and use of -webkit-prefixed things even when standard alternatives are available is rampant and purposeful. And when you ask these people to develop to standards they just laugh at you.