@import url("https://fonts.googleapis.com/css2?family=Exo:wght@600&display=swap");

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}
html {
  height: 100%;
}

body {
  width: 100vw;
  height: min(100vh, 100%);
  font-family: "Exo", Arial, sans-serif;
  background-color: #557;
  color: #fff;
  display: flex;
  justify-content: center;
  align-items: center;
}

body {
  background: linear-gradient(to right, purple 0, purple 100%) 0 0 /
      max(0px, 320px - 100%) 1px,
    linear-gradient(to right, blue 0, blue 100%) 0 0 / max(0px, 760px - 100%)
      1px,
    linear-gradient(to right, green 0, green 100%) 0 0 / max(0px, 1024px - 100%)
      1px;
  background-color: red;
}