<div></div>

html {
  height: 100%;
}
body {
  background: #aaa;
  height: 100%;
  margin: 0;
}
div {
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100%;
}
div:before {
  content: "";
  height: 0;
  width: 0;
  display: inline-block;
  border: 1em solid #fff;
  border-top-color: #f00;
  border-bottom-color: #f00;
}