CSS

center content

display: flex;
justify-content: center;

center div content

**HTML**
<div class="container">
  <div class="child"></div>
</div>

**CSS**
.container {
  position: relative;
}

.child {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}

center div content css

/*
if selector has a width defined
*/
#line {
 width: 400px;
 margin-left: auto;
 margin-right: auto;
}