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

body {
  display: flex;
  flex-direction: row;
}

#menu {
  flex-basis: 20%;
  background-color: #f1f1f1;
  padding: 20px;
}

#contenu {
  flex-basis: 80%;
  padding: 20px;
}