How to align a <div> to the middle (horizontally/width) of the page

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;
}
<div id="outPopUp"></div>

<body>
    <div style="width:800px; margin:0 auto;">
        centered content
    </div>
</body>

Flexbox solution is the way to go in/from 2015. justify-content: center is used for the parent element to align the content to the center of it.

HTML

<div class="container">
  <div class="center">Center</div>
</div>

CSS

.container {
  display: flex;
  justify-content: center;
}

Output

.container {
  display: flex;
  justify-content: center;
}
.center {
  width: 400px; 
  padding: 10px;
  background: #5F85DB;
  color: #fff;
  font-weight: bold;
  font-family: Tahoma;
}
<div class="container">
  <div class="center">Centered div with left aligned text.</div>
</div>