* {
  box-sizing: border-box;
}

body {
  height: 100dvh;
  margin: 0;
}

html::before,
html::after,
body::before,
body::after {
  content: "";
  position: fixed;
}

html::before,
html::after {
  width: 100%;
  height: 20px;
  left: 0;
}

body::before,
body::after {
  height: 100%;
  width: 20px;
  top: 0;
}

html::before {
  top: 0;
  background: oklch(from red l c h / 0.5);
}

html::after {
  bottom: 0;
  background: oklch(from green l c h / 0.5);
}

body::before {
  left: 0;
  background: oklch(from blue l c h / 0.5);;
}

body::after {
  right: 0;
  background: oklch(from yellow l c h / 0.5);;
}