[aristus]

Yes, you've found the five-word phase that mentions that there are extensions in the selector engine. There is no documentation on which things are, or are, not, extensions.

Now that we've gone all the way down the pedantry slide, have you anything of value to contribute, or was that it?

[freakwit]

You're being disingenuous here. Anybody looking for information about jQuery's selectors will google "jquery selectors" and find that five-word phrase front and center. Or take a look at the jQuery front page and hover over "CSS3 Compliant" to see "Supports CSS 1-3 selectors and more!".

If you want to pick on something that claims compatibility, try Sizzle itself - http://sizzlejs.com/. In that case, you'll find a summary of alterations right at the top of its documentation https://github.com/jquery/sizzle/wiki .