<div class="box"></div>

html, body {
  margin: 0;
}
div {
  width: 100vw;
  height: 100vh;
  background-color: white;
}
.box:after {
  content: ' ';
  border-top: 100vh solid #008ce6;
  border-right: 100vw solid transparent;
  width: 0;
  position: absolute;
}