<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <style>
#x {
  width: 200px;
  height: 200px;
  border: 40px;
  border-style: solid;
  border-color: transparent;
  background-image: repeating-linear-gradient(to bottom right, white, black, white 30px);
}
  </style>
</head>
<body>
  <div id="x"></div>
</body>
</html>