Always Center

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

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

// within container

.Center-Container {
  position: relative;
}

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

// within viewport

.Absolute-Center.is-Fixed {
  position: fixed;
  z-index: 999;
}

// offsets

.Absolute-Center.is-Right {
  left: auto; right: 20px;
  text-align: right;
}

.Absolute-Center.is-Left {
  right: auto; left: 20px;
  text-align: left;
}