<header></header>
<main></main>

* {
  margin: 0;
  padding: 0;
}

header {
  height: 20vh;
  background-color: steelblue;
  position:relative;
}

header:after {
  box-shadow: 0 1vh 1vh teal;
  content: '';
  position: absolute;
  width: 100%;
  height: 100%;
}

main {
  height: 80vh;
  background-color: ivory;
}