CSS: center element vertically and horizontally

HTML

    
center

CSS for centering the element vertically and horizontally

.center-vertically-horizontally {
  display: flex;
  align-items: center; // vertically center 
  justify-content: center; // horizontally center
  text-align: center; // horizontally center text that breaks into multiple lines
}
#container-demo
{
    width:200px;
    height:300px;
    background:#eee;

}
#center-item-demo
{
    width:150px;
    height:50px;
    background:green;
    margin: auto;
    justify-content: center;
}