Vertical and horizontally centered overlay using just css (flexbox)

gistfile1.txt

.overlay-mask {
  display: -webkit-box; /* OLD - iOS 6-, Safari 3.1-6 */
  display: -moz-box; /* OLD - Firefox 19- (buggy but mostly works) */
  display: -ms-flexbox; /* TWEENER - IE 10 */
  display: -webkit-flex;
  display: flex;

  -webkit-box-align: center;
  -webkit-flex-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;



  -webkit-box-orient: vertical;
  -moz-box-orient: vertical;
  -webkit-box-direction: normal;
  -moz-box-direction: normal;
  -webkit-flex-direction: column;
  -ms-flex-direction: column;
  flex-direction: column;

  position:fixed;
  height: 100%;
  width: 100%;
  top: 0;
  z-index: 999;
  background-color: rgba(0, 0, 0, 0.5);
  padding: 6px;
  -webkit-box-sizing: border-box;
  -moz-box-sizing: border-box;
  box-sizing: border-box;
}

.overlay-content {
    padding: 5px;
    min-height: 10px;
    min-width: 10px;
    margin-top: 100px;
    margin: auto;
    background-color: #fff;
    -webkit-border-radius: 12px;
    -moz-border-radius: 12px;
    border-radius: 12px;
    border: 1px solid #a5a5a5;
    position: relative;
    -webkit-align-self: center;
    align-self: center;
    -webkit-align-content: center;
    align-content: center;
}