style.css

body {
  background-color: gray;
  height: 3000px;
  padding-top: 75px;
}

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

.navbar {
  width: 100%;
  height: 75px;
  background-color: red;
  position: fixed;
  top: 0;
}

.container {
  width: 1200px;
  height: 600px;
  background-color: aquamarine;
  margin-top: 20px;
  margin: 0 auto;
  position: relative;
}

.container .content {
  width: 920px;
  height: 480px;
  background-color: yellow;
  margin: auto;
}

.box {
  display: inline-block;
  width: 250px;
  height: 250px;
  color: white;
  font-size: 50px;
  text-align: center;
  background-color: red;
}

.box1 {
  background-color: green;
}
.box2 {
  background-color: black;
  position: absolute;
  top: 0;
  left: 0;
}
.box3 {
  background-color: blue;
}

#sticky {
  width: 100%;
  height: 75px;
  background-color: purple;
  position: sticky;
  top: 0;
  z-index: 99;
}