Lots of things get weird in edge cases with SVGs. E.g., one thing that keeps biting me is that Firefox always snaps x="X" y="Y" coordinates on <use> elements to the nearest CSS pixel (even if the pixel size is very large), but allows transform="translate(X,Y)" to keep its full subpixel precision. I've taken to double-checking all my SVGs at 3000% zoom just to spot all the seams that implementations can create.