How To Phish, Protect Your Email, and Defeat Copy-And-Paste with CSS: Here’s a sneaky but facepalm-obvious way to mess with users. Basically, you embedd random characters and other text inside of your real text, then hide the fake text with CSS. It looks fine in a browser, but copies-and-pastes with all the crap.
A publisher could generate, on the server side, a new random mess of HTML and CSS that would render their text uncopyable. This also has the side-effect of making your pages impossible for search engines to index sensibly; it’s an easy way to keep your information human-readable but cloaked from Google’s all-seeing Sauronic eye.
He details some other, more nefarious uses as well.