CSS: Responsive way to center a fluid div (without px width) while limiting the maximum width?

Question

I\'ve seen a million questions about how to center a block element and there seem to be a couple popular ways to do it, but they all rely on fixed pixels widths. Then either

Answer 1

EDIT :
http://codepen.io/gcyrillus/pen/daCyu So for a popup, you have to use position:fixed , display:table property and max-width with em or rem values :) with this CSS basis :

#popup {
  position:fixed;
  width:100%;
  height:100%;
  display:table;
  pointer-events:none;
}
#popup > div {
  display:table-cell;
  vertical-align:middle;
}
#popup p {
  width:80%;
  max-width:20em;
  margin:auto;
  pointer-events:auto;
}