This would be difficult with pure css because the citations can be expanded, which in turn pushes any colliding citations slightly further down.
On firefox I'm not seeing them get pushed down, I'm seeing them get covered. I suspect the citation implementation isn't very polished. Looks good though.
I found it hideous. Trying to highlight paragraphs doesn't work either strangely it ends up selecting other page content which makes copying a pain
The citations don’t appear at all for me on mobile safari in private mode.