CSS Viewports

body::before {
  background-color: #000;
  color: #fff;
  content: "XS";
  font-size: 0.5em;
  top: 0;
  left: 0;
  padding: 0.2em;
  position: fixed;
  z-index: 9999999;
}
@media (min-width : 768px) { body::before { content: "SM"; }}
@media (min-width : 992px) { body::before { content: "MD"; }}
@media (min-width : 1200px) { body::before { content: "LG"; }}