I've been surprised by how performant you can get with CSS transitions and a will-change property. But good grief is it difficult.
On one hand I understand why a transition on `left` isn't hardware accelerated but a `translate3d` is, but on the other it seems crazy that we expect people to think and know this kind of stuff when they just want to do something very simple.
That's was my intention when I proposed will-changed. I was trying to get the Firefox OS home screen slide transitions to be performant without hard coding it in the Gecko layout engine.
I agree with you. Performant animations with JavaScript are generally painful. CSS helps, but can be limited. It can also be a bit fiddly, especially when you want to chain animations together. Perhaps the web animations API will solve this as it matures.
On one hand I understand why a transition on `left` isn't hardware accelerated but a `translate3d` is, but on the other it seems crazy that we expect people to think and know this kind of stuff when they just want to do something very simple.