body {
  margin: 0;
  font-family: Arial, sans-serif;
}

nav {
  background: gray;
  padding: 10px;
  text-align: center;
}

nav div {
  display: inline-block;
  background: darkgray;
  color: white;
  padding: 8px 12px;
  margin: 4px;
}

.layout1 {
  padding: 10px;
}

.box {
  width: 100%;
  height: 100px;
  background: lightblue;
  margin: 10px 0;
}

@media (min-width: 600px) {
  .box {
    width: 48%;
    display: inline-block;
  }
}

@media (min-width: 900px) {
  .box {
    width: 30%;
  }
}