[podperson]

That bit certainly surprised me. It seems like a simple optimization to scan a selector for the most efficient parts and use that as your starting point (with ids being a no-brainer).

Obviously given the selector "div #foo a {}" (obviously a dumb selector, but...) it would be smarter to look for #foo and verify it was inside a div than get every div and search for a #foo. And by extension "div .foo a {}" (assuming .foo really is way more efficient than div, which again I find hard to believe given there's native support for getElementsByTagType).

It seems to me that the effectiveness of using classes probably stems from browsers optimizing against use cases like "OMG folks are using Dijit and hanging twelve classes off every div". There's no reason I can see why class selectors ought to be especially efficient, especially if -- say -- your markup has lots of classes and relatively few attributes (and isn't that more common these days?).

We should also probably differentiate between CSS selectors as affects page rendering and CSS selectors as affects grabbing a bag of nodes using jQuery $() or document.querySelectorAll().