<!DOCTYPE html>
<html>
<style>
@keyframes anim {
  from { background-color: rgba(0, 0, 0, 0); }
  to { background-color: rgba(255, 0, 0, 255); }
}
body {
  animation: anim 100s;
  width: 100vw;
  height: 100vh;
}
</style>
