CSS: center element within a <div> element

.flex-container{
  display: -webkit-box;  /* OLD - iOS 6-, Safari 3.1-6, BB7 */
  display: -ms-flexbox;  /* TWEENER - IE 10 */
  display: -webkit-flex; /* NEW - Safari 6.1+. iOS 7.1+, BB10 */
  display: flex;         /* NEW, Spec - Firefox, Chrome, Opera */
  
  justify-content: center;
  align-items: center;
  
  width: 400px;
  height: 200px;
  background-color: #3498db;
}

.inner-element{
  width: 100px;
  height: 100px;
  background-color: #f1c40f;
}

<div class="flex-container">
  <div class="inner-element"></div>
</div>

.flex-container{
  position: relative;
  
  /* Other styling stuff */
  width: 400px;
  height: 200px;
  background-color: #3498db;
}

.inner-element{
  position: absolute;
  left: 50%;
  top: 50%;
  
  transform: translate(-50%,-50%);
  /* or 3d alternative if you will add animations (smoother transitions) */
  transform: translate3d(-50%,-50%,0);

  /* Other styling stuff */
  width: 100px;
  height: 100px;
  background-color: #f1c40f;
}

<div class="flex-container">
  <p>Other inner elements like this follows the normal flow.</p>
  <div class="inner-element"></div>
</div>

#parent {
    text-align:center;
    background-color:blue;
    height:400px;
    width:600px;
}
.block {
    height:100px;
    width:200px;
    text-align:left;
}
.center {
    margin:auto;
    background-color:green;
}
.left {
    margin:auto auto auto 0;
    background-color:red;
}
.right {
    margin:auto 0 auto auto;
    background-color:yellow;
}

<div id="parent">
    <div id="child1" class="block center">
        a block to align center and with text aligned left
    </div>
    <div id="child2" class="block left">
        a block to align left and with text aligned left
    </div>
    <div id="child3" class="block right">
        a block to align right and with text aligned left
    </div>
</div>