JSFiddle - React, Tailwind, and code Playground

SCSS

body {
  width: 100vw;
  height: 100vh;
  position: relative;
  &::before {
    content: "";
    display: block;
    position: fixed;
    width: 100%;
    background: red;
    height: 0;
    z-index: 0;
    animation-name: expand;
    animation-duration: 10s;
  }
}

@keyframes expand {
    from { height: 0; }
    to { height: 100%; }
}