body {
  margin: 0;
  display: flex;
  flex-direction: column;
  height: 100vh;
}
header nav ul {
  margin: 0;
  padding: 0;
  list-style: none;
  display: flex;
  flex-direction: row;
}
header nav ul li {
  text-align: center;
  flex-grow: 1;
  line-height: 40px;
}
header nav ul li:hover {
  background-color: red;
}
main {
  flex-grow: 1;
  overflow-x: hidden;
  overflow-y: auto;
}
footer {
  flex-basis: 40px;
}