Responsive Web Design

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

body {
  padding: 10px;
  margin: auto;
}

header,
.left,
.right {
  padding: 10px;
  margin-top: 10px;
}

header {
  height: 80px;
  background-color: #f1f5ff;
}

.left {
  height: 250px;
  background-color: #d4d7ff;
}

.right {
  height: 250px;
  background-color: #ffd4d7;
}

@media (min-width: 767px) {
  .left {
    width: 29%;
    float: left;
  }
  .right {
    width: 69%;
    float: right;
  }