Enabling pixel grabbing in SVG just like Canvas

At SVG Open, there was discussion today about adding Canvas's pixel
grabbing APIs to SVG, with claims that SVG implementations don't
necessarily retain a rendering surface. It now occurs to me that there is a
CSS property ('enable-background') in the SVG 1.1 spec just for this
purpose:

* http://www.w3.org/TR/SVG/filters.html#AccessingBackgroundImage

Pixel grabbing APIs in SVG could require that 'enable-background' is placed
on the outermost <svg> element as a hint to the implementation that it
needs to retrievable retain pixel values for image.

Note that implementations pretty much have to implement saved rendering
surfaces to do group opacity even if they haven't implemented filter
effects yet.

Jon Ferraiolo
IBM