One word: (Score 1)

Consistency.

This applies to "Future Browsers" as a group. Get together and make rendering and APIs consistent. Please. I really don't even care if you follow the W3 "standard" (which is certainly far from optimal). Just make it all consistent, guys. Make it so I can learn the rendering model and APIs and be done with it, instead of banging my head against the wall trying to figure out why browser X has a different behavior than browser Y and which one is correct and what I should do about it.