body {
  margin: 0;
}

h1 {
  text-align: center;
  margin: 0;
  padding: 10px;
}

main {
  width: 90%;
  margin: auto;
}

div {
  border: 1px solid black;
  display: inline-block;
  width: 30%;
  padding: 0.5em;
  box-sizing: border-box;
  margin: 1.5%;
  background-color: #F5FFF5;
}

header {
  width: 100%;
  background-color: lightgray;
}

nav {
  width: 100%;
  background-color: black;
  color: white;
}

@media only screen and (max-width:900px) {
  main > div {
    width: 45%;
    background-color: #FFFFEE;
  }
}

@media only screen and (max-width: 600px) {
  main > div {
    width: 100%;
    margin-left: 0;
    margin-right: 0;
  }
}

nav ul li {
  width:100%;
}

nav ul li a {
  border: 1px solid gray;
}

header h1 {
  text-align: left;
}

nav a {
  padding: 1rem;
  display: block;
  text-decoration: none;
  color: white;
  box-sizing: border-box;
}

nav a:hover {
  background-color: red;
}