I’ve noticed that the official CSS reference page is missing links to some CSS selector pages, like cursor and user-select. I assume this is because their specification is still in draft form (although not in the case of cursor). If the CSS reference page shouldn’t link to these less stable (but still useful) selectors, I suggest there be a reference page for newer but mostly stable selectors. This page could be linked to in the primary CSS reference page.
I personally find the CSS reference page extremely helpful when looking up a selector. However, with some useful selectors missing, I feel some selectors may be harder to find than they could be.