It's difficult, because you cannot see the pixels in an HTML element's rendering with JavaScript. If you could copy a link to a canvas, then you could see if it

It's difficult, because you cannot see the pixels in an HTML element's rendering with JavaScript. If you could copy a link to a canvas, then you could see if it has been previously visited, for example. So, canvases would not work. I may be possible with SVG filters or WebGL, but that also has security implications. It's possible to read cross-origin content using timing attacks and filters. http://www.contextis.com/files/Browser_Timing_Attacks.pdf http://www.contextis.com/files/Browser_Timing_Attacks.pdf