A technique for creating a non-graphical CAPTCHA using CSS alone is described here.
The technique relies on the fact that HTML element borders can be individually styled using CSS and makes use of the CSS float attribute.
The technique is presented largely as an interesting demonstration of what can be accomplished using CSS. The CAPTCHA in its current form could be solved programmatically with no great difficulty. It is possible to create a similar CAPTCHA that would be a great deal harder to crack but I am not sure it is worth the effort.
In any case, I am curious to have some feedback and - should anyone care - see any attempts at writing code to solve it.