I'll put python/java/php sources up once I got the specs for the proxy more set (currently still could get some changes that could break the whole functionality), but in short it provides the images either base64 encoded jsonp if browser doesn't support XHR2, if it does, it provides it as an image with CORS enabled (prefered as it is slightly smaller than base64 encoded).

For the HTML, it just checks a simple charset check and does the same as for images.

There are plenty of examples of the script embedded on the page though, and you can always download a copy from github and try it on your own page meanwhile.

copy of the proxy source available at https://github.com/niklasvh/html2canvas-proxy now