There's something like Acid3, but it doesn't give you information as to what failed when you get a less-than-perfect score. (Test: http://acid3.acidtests.org/ | Information: http://www.webstandards.org/action/acid3/)

There are also all those other browser benchmarks (e.g. http://peacekeeper.futuremark.com/), but those include things other than the layout engine, such as javascript performance, webGL, etc. You could try poking around a few of them and only looking at the CSS and DOM sections though (http://web.basemark.com/).