From http://coding.smashingmagazine.com/2013/08/09/absolute-horizontal-vertical-centering-css/

Absolut centering

.Absolute-Center {
  margin: auto;
  position: absolute;
  top: 0; left: 0; bottom: 0; right: 0;
}

/****************************************/

.Absolute-Center {
  width: 50%;
  height: 50%;
  overflow: auto;
  margin: auto;
  position: absolute;
  top: 0; left: 0; bottom: 0; right: 0;
}

/**************************************/

.Absolute-Center.is-Fixed {
  width: 50%;
  height: 50%;
  overflow: auto;
  margin: auto;
  position: fixed;
  top: 0; left: 0; bottom: 0; right: 0;
  z-index: 999;
}

/**************************************/

.Absolute-Center.is-Responsive {
  width: 60%; 
  height: 60%;
  min-width: 400px;
  max-width: 500px;
  padding: 40px;
  overflow: auto;
  margin: auto;
  position: absolute;
  top: 0; left: 0; bottom: 0; right: 0;
}

/**************************************/

.Absolute-Center.is-Right {
  width: 50%;
  height: 50%;
  margin: auto;
  overflow: auto;
  position: absolute;
  top: 0; left: auto; bottom: 0; right: 20px;
  text-align: right;
}

/*************************************/

.Absolute-Center.is-Overflow {
  width: 50%;
  height: 300px;
  max-height: 100%;
  margin: auto;
  overflow: auto;
  position: absolute;
  top: 0; left: 0; bottom: 0; right: 0;
}

/**************** IMAGE **************/

.Absolute-Center.is-Image {
  width: 50%;
  height: auto;
  margin: auto;
  position: absolute;
  top: 0; left: 0; bottom: 0; right: 0;
}

/************************************/

.Absolute-Center.is-Variable {
  display: table;
  width: 50%;
  overflow: auto;
  margin: auto;
  position: absolute;
  top: 0; left: 0; bottom: 0; right: 0;
}

/*************************************/

.is-Negative {
        width: 300px;
        height: 200px;
        padding: 20px;
        position: absolute;
        top: 50%; left: 50%;
        margin-left: -170px; /* (width + padding)/2 */
        margin-top: -120px; /* (height + padding)/2 */
}

/*************************************/

.is-Transformed { 
  width: 50%;
	margin: auto;
	position: absolute;
	top: 50%; left: 50%;
	-webkit-transform: translate(-50%,-50%);
    -ms-transform: translate(-50%,-50%);
        transform: translate(-50%,-50%);
}

/****************************************/

<div class="Pos-Container is-Table">
  <div class="Table-Cell">
    <div class="Center-Block">
    <!-- CONTENT -->
    </div>
  </div>
</div>

.Pos-Container.is-Table { display: table; }
.is-Table .Table-Cell {
  display: table-cell;
  vertical-align: middle;
}
.is-Table .Center-Block {
  width: 50%;
  margin: 0 auto;
}

/***********************************/

.Pos-Container.is-Flexbox {
  display: -webkit-box;
  display: -moz-box;
  display: -ms-flexbox;
  display: -webkit-flex;
  display: flex;
  -webkit-box-align: center;
     -moz-box-align: center;
     -ms-flex-align: center;
  -webkit-align-items: center;
          align-items: center;
  -webkit-box-pack: center;
     -moz-box-pack: center;
     -ms-flex-pack: center;
  -webkit-justify-content: center;
          justify-content: center;
}