Separation of Content and Presentation with HTML and CSS

Almost every day I come across sites which have missed this vital point entirely. They think that just by adding CSS to a site, it's suddenly now much better. This is not the case, and I admit I've been in this category myself in the past.

The whole concept of HTML and CSS is to separate the content of a site, from the presentation. To layer it. This means that there should be absolutely NO content in the CSS, and absolutely NO presentation in the HTML.