@keyframes background {
    0% {
      background-position: 50% 0%
    }
    50% {
      background-position: 50% 100%
    }
    100%{
      background-position: 50% 0%
    }
  }
  
  html {
    height: 100%;
  }
  body {
    height: 100%;
    background: linear-gradient(225deg, #ff0000, #00ffff);
    background-size: 300% 300%;
    animation: background 10s linear infinite;
  }