Predictability in UI design

So today I had the dubious pleasure of working all afternoon on UI design in Cocoa, and then UI design in Rails/CSS/HTML.?
Going from one to the other really highlights some of the problems in doing web-based design!

My big issue with writing in HTML/CSS is? predictability. I do not? enjoy wondering if something is going to render properly, or if a particular CSS attribute is going to work the way it ought to.

Maybe I’m doing it wrong. Maybe CSS doesn’t suck as bad as I seem to think. But until I can say “you, the content box! Size yourself to fit around the content inside you and then position yourself in the center of the screen” just that easily…