Implement the border-image-outset property (and the equivalent behavior for masks).
Created attachment 106906 [details]
Fixed in r94912.
This broke media/video-zoom-controls.html by adding a 0x30 DIV to each video (containing "00:00", presumably the time counter for the video); see bug 67913.
Was this effect intentional or is there follow-up work planned to address this?