body {
  margin: 0px;
  font-family: Arial;
  line-height: 20px;
}
#header {
  height: 20px;
  background: #222;
  color: white;
}
#content {
  min-height: calc(100vh - 40px); substract 40px since 20 are from header and 20 from footer
}
#footer {
  height: 20px;
  background: #222;
  color: white;
}