To be fair, centering with flex doesn't require the parent to have any particular positioning set. The only reason you need to set the width/height is because the centering happens inside 0 size box, which wouldn't happen if your problem is centering something inside existing box.

The flexbox method does work with an unknown height. The point is that the container has to actually have a height (one way or another) that is greater than the height of the aligned element, otherwise vertical alignment is all equal anyway! This is as true of the flexbox approach as it is in your example (which explicitly sets the containing 'section' element's height).