It's worse than we thought.
I always like to build a local test.
Download www.eklhad.net/csstest.zip and review.

over.css: the selectors from stackoverflow.com, this is their css file.
csstest: the file to browse.

Browse with db3 of course.
eb$qs$start comes back in jig time, which suggests the slowdown is in traversing and testing all the nodes, not in the parsing.
This test file only has 6 nodes, as shown by dumptree(document).
Problems listed below.

1. There is a syntax error.
Their software doesn't allow a classname to start with a dash, and yet it can, I guess.
You see the syntax error with db3.

2. Their program stops at the first error. So it only did part of the list.

3. This error occurs at 471, out of 3198.
If it had gone all the way through, on stackoverflow.com, this routine would take 2 times 3198 / 471 = 13.5 minutes.
So we can rightfully think of a 13.5 minute startup cost just to browse this site.
Obviously we can't live with that.

There are so many issues I almost think we should rewrite it from scratch, either in C or js.
Obviously I'd use theirs as a guide, but lordy! IDK

Karl Dahlke