Center middle a Div

centermiddlediv.css

/*
  When changing the width and height, change the margin to a negative half sized value, and line height 
/*

#centermiddle {
  margin: -100px 0 0 -100px;
  display: block;
  border:1px solid red;
  width: 200px ;
  height: 200px;
  position: fixed;
  left: 50%;
  top: 50%;
  line-height: 200px;
  text-align: center
}