5.css

h1 {
  text-align: center;
}

p {
  width: 85%;
}

.smartphone_sample {
  text-align: center;
}

html, body {
  margin: 0;
  padding: 0;
}

header {
  background-color: yellow;
  width: 100%;
  height: 15%;
}

nav {
  width: 15%;
  height: 890px;
  float: left;
  display: inline-block;
  background-color: orange;
}

section {
  width: 85%;
  height: 70%;
  float: left;
  display: inline-block;
  background-color: darkturquoise;
}

footer {
  width: 100%;
  height: 15%;
  clear: both;
  background-color: mediumpurple;
}