<div></div>

div {
  background: #000;
  width: 10px;
  height: 10px;
  
  box-shadow:
    10px 10px  red,
    -10px 10px orange;
}



html, body {
  height: 100%;
}
body {
  display: flex;
  justify-content: center;
  align-items: center;
}