My ideal world would have exactly one browser engine, with everyone running the same version. That would allow skipping the entirely of the testing-after-development phase, and to use everything that engine provides -- a huge improvement to current status quo. And if it was nailed down in such a way that I could specify "I designed this to look and work right with version 12.34" (similar to what Microsoft is doing, incidentally, with their IE version compatibility tagging, and what Android is doing, with the manifest version specification), I would then simply trust that future versions of browsers retain compatibility to past versions, and solve this thorny problem for me.
The difference in our mindsets could hardly be greater. I think browsers grew up a huge deal when the acid tests arrived, because they provided a nice score that anybody could check and solved a lot of incompatibility problems back in the day when browser vendors rushed to compete against each other to beat their rivals' scores. So I would credit those tests, and the long-awaited death of IE6 and now IE7 for making our lives quite easy these days.