A completely obvious idea that I should have thought of earlier, but so should the search engines.

An advanced search should allow querying on presentational properties of particular pages. For example, I want to search for portfolio sites with black backgrounds. You can search for “#000000”, but since CSS files are not indexed, this only shows pages that contain that particular markup – and most good web designers put that sort of thing in a CSS file nowadays.

This is just one example. I could want to search for pages with a certain layout, with certain placement of images, etc. I can’t do any of that with current search engines.