How do I centre absolutely positioned content of unknown width?

Question

Before someone asks me why the hell I would want to do this let me come straight out and tell you. That way one of you clever peeps out there can tell me a far better way...

Answer 1

To go off of Samec's answer, you can also use this to center an absolute position element vertically and horizontally of unknown dimensions:

#viewer_div {
  position: relative;
}
#viewer_div img {
  position: absolute;
  left: 50%;
  top: 0%;
  transform: translate(-50%, 50%);
}