Hiding images in highlighted text

Self-described "creative coder" Neal Agarwal has come up with a method for hiding images in text that only appear when you highlight the words; I'm guessing he's using some kind of character-by-character "highlight" attribute in CSS/HTML5, but I'm not really certain. It's a super fun example of steganography, though, and would be really fun for some kinds of online puzzles, or spoiler reveals in posts, etc.

(via Dan Hon)