header {
 background-image:url("bugonia.jpg");
  background-size:100% 100%;
  aspect-ratio:8412/2455;
  box-sizing:border-box;
 text-align:center;
  padding-top:3%;
  font-size:7vw;
}
section {
  display: -webkit-flex;
  display: flex;
}
nav {
  -webkit-flex: 1;
  -ms-flex: 1;
  flex: 1;
  background: #ccc;
  padding: 20px;
}
article {
  -webkit-flex: 3;
  -ms-flex: 3;
  flex: 3;
  background-color: #f1f1f1;
  padding: 10px;
}