Comment 4 for bug 481605

So it sounds like this is about looking at the visible portions of the svg image. As a first iteration, any elements that are totally obscured by other (opaque) elements can be totally removed from the image.

Next, for merging elements as this bug requests, you'd have to look for partial overlaps, find intersections and replace the intersecting elements. One problem here is that replacing a few overlapping rects (for example) with an equivalent path could end up actually making the svg source larger. The rendering time after replacement could be faster or slower, not sure.

Since this can alter the document pretty severely it should be off by default.