You may have already guessed the answer is "no" due to the lack of replies. The issue is the CSS doesn't know what part of the page is within the viewport. Anot

You may have already guessed the answer is "no" due to the lack of replies. The issue is the CSS doesn't know what part of the page is within the viewport. Another issue is a table of contents should be in the page before the contents themselves and you generally can't change style of an element based on the state of a later element (I qualify it with "generally" because there are some possibilities with the newish :focus-within pseudoclass).