In particular, this causes shadows in Blink to render incorrectly for printing.

For Blink's shadows to render correctly, this needs to be supported when drawing, at least:
- text for text shadows
- rects, rounded rects, and other paths (e.g. the rect-with-rounded-hole path used for inner shadows) for box shadows

I have not yet investigated the possible types of shadows that can arise from SVG.

Other browser vendors' backends appear to implement this feature by embedding bitmaps in the resulting PDF (e.g. Safari's output for a blue text shadow includes a solid blue rectangle and a mask image which shows the blurred text).

A past attempt at this for text, at least, is at http://crrev.com/14854014, but that has stalled.