.container {
  position:relative;
  width:250px;
  height:250px;
  border:1px solid #000;
}

.centerBox {
  top:50%;
  left:50%;
  padding:10px;
  transform:translate(-50%, -50%);
  color:#fff;
  border-radius:3px;
}

.centerBox.absolute {
  position:absolute;
  background:#18ab28;
}

.centerBox.fixed {
  position:fixed;
  background: linear-gradient(-20deg, transparent, #f0a24f);
}

.centerBox:before {
  content:'';
  display:inline-block;
  position:absolute;
  top:0;
  left:0;
  width:8px;
  height:8px;
  background:red;
  border-top-left-radius:3px;
}