<!doctype html>
<head>
<link rel="stylesheet" href="/slow-response">
<script>
  const params = new URLSearchParams(window.location.search);
  if (params.has('red')) {
    document.documentElement.style.background = "red";
  } else {
    document.documentElement.style.background = "blue";
  }
</script>
<style>
  html {
    margin: 0;
  }
</style>
</head>
<body>
</body>