html, body {
  height: 100%;
}

* {
  margin: 0;
  padding: 0;
}

section {
  height: 100%;
  display: flex;
  flex-direction: column;
}

.header {
  flex: 0 1 auto;
  width: 100%;
  text-align: center;
  background-color: #f3f3f3;
  padding-top: 5px;
  padding-bottom: 5px;
}

.header a {
  color: red;
  text-decoration: none;
}

.page {
  flex: 1 1 auto;
  height: 100%;
}