Captcha for Humans
Please enter the SUM of all the GREEN FIGURES (not letters) below (that's twice the same image, just with a different HTML background - the Data-URI-inlined GIF background is transparent):
The two sums are: 4 and 21... for the same Captcha image!
By just changing the HTML background color (mouse cursor hovering, previous state or input or shared secret) used for the transparent GIF Captcha image we can make something simple for humans become difficult or even completely impossible for robots.
HTML and GIF are served with one single request: the picture is generated on-the-fly and embedded into the HTML code by using the base64 encoding (look at the HTML source code).