使用Javascript从浏览器屏幕截图？

I guess this is an odd one, and the answer is most likely it is not possible since it would represent a security breach; but I am looking for a way - if any - to get a screendump of content inside the browser. I don't need the entire window, but actually just need to dump the rendered state of an tag. It need to be cross platform and cross browser compatible.I guess this is an odd one, and the answer is m