Good CSS architecture practices
One thing we can be clear on is that using a flat hierarchy of class-based selectors, as is the case with ECSS, provides selectors that are as fast as any others.
What does this mean?
For me, it has confirmed my believe that it is absolute folly to worry about the type of selector used. Second guessing a selector engine is pointless as the manner selector engines work through selectors clearly differs. Further more, the difference between fastest and slowest selectors isn't massive, even on a ludicrous DOM size like this. As we say in the North of England, There are bigger fish to fry.
Since documenting my original results, Benjamin Poulain, a WebKit Engineer got in touch to point out his concerns with the methodology used. His comments were very interesting and some of the information he related is quoted verbatim below:
By choosing to measure performance through the loading, you are measuring plenty of much much bigger things than CSS, CSS Performance...