How to align a to the middle (horizontally/width) of the page [duplicate]

Answer 1

position: absolute and then top:50% and left:50% places the top edge at the vertical center of the screen, and the left edge at the horizontal center, then by adding margin-top to the negative of the height of the div, i.e., -100 shifts it above by 100 and similarly for margin-left. This gets the div exactly in the center of the page.

#outPopUp {
  position: absolute;
  width: 300px;
  height: 200px;
  z-index: 15;
  top: 50%;
  left: 50%;
  margin: -100px 0 0 -150px;
  background: red;
}