CSS

animated background css

/*
	use @keyframes and the animation property
	see this code running here: https://codepen.io/hylobates-lar/pen/qBbQeON
*/
html, body {
  width: 100%;
  height:100%;
}

body {
    background: linear-gradient(-45deg, #ee7752, #e73c7e, #23a6d5, #23d5ab);
    background-size: 400% 400%;
    animation: gradient 15s ease infinite;
}

@keyframes gradient {
    0% {
        background-position: 0% 50%;
    }
    50% {
        background-position: 100% 50%;
    }
    100% {
        background-position: 0% 50%;
    }
}

animation background css

@keyframes gradient {
    0% {
        background-position: 0% 50%;
    }
    50% {
        background-position: 100% 50%;
    }
    100% {
        background-position: 0% 50%;
    }
}
body{
    width: 100vw;
    height: 100vh;
}
div{
    transition: all .3s ease-in-out;
    background: linear-gradient(-45deg, #ee7752, #e73c7e, #23a6d5, #23d5ab);
    background-size: 400% 400%;
    animation: gradient 15s ease infinite;
    height: 500px;
    width: 500px;
}

html animated background

    <video id="video" autoplay="true" loop muted>
      <source src="background.mp4" type="video/mp4"/>
    </video>