That's intelligent. CSS taking care of CSS.

This part was specially interesting:

/* These may not be "wrong", but let's bug you anyway. */

b:after, i:after, blink:after, marquee:after { background: green; content: 'Should you be using strong or em?'; }

Made a bookmarklet for it here: http://steve.io/debugcss/

Here is another one - http://www.red-root.com/sandbox/holmes/

HN Discussion - http://news.ycombinator.com/item?id=2489546

I was not aware that you could inject content with CSS. Cool.

Any links to an example?