Footer in bottom

by head__space

HTML

<body>
   <header>green</header>
   <footer>black</footer>
</body>

CSS

html,
body {
  height: 100%;
}
body {
  display: flex;
  flex-direction: column;
}
header {
  flex: 1 0 auto;
}
footer {
  flex: 0 0 auto;
  background: #1D1D1D;
}