nav {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    background-color: #333;
    color: #fff;
    padding: 10px;
    text-align: center;
  } 
  
header {
  color: yellow;
  position: absolute;
  top: 20px;
  left: 0;
  right: 0;
  text-align: center;
  width: 100%;
}
img {
  max-width: 100%;
  height: auto;
  display: block;
  margin: 0 auto;
}
  
body {
  background-color: #333;
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
}