[sdrothrock]

If the author also used the checked/unchecked disabled states, that might open up more possibilities with four values to with rather than just two. :)

[inopinatus]

You can also use required checkboxes to make form elements (and their contents) in entirely separate DOM subtrees match the :invalid state, with hilarious opportunities for CSS.

[simonbarker87]

Throw in the indeterminate state and he’s got another state with slight visual difference, might be able to add a little texture/depth maybe.

[verisimi]

Maybe he doesn't know about that .. someone ought to drop him an email.