html {
  box-sizing: border-box;
}

body {
  margin: 50px;
}

*, *:before, *:after { 
  box-sizing: inherit; 
}

div {
  width: 90%;
  padding: 5%;
  margin: 5%;
}

.div1 {
  background: thistle;
}

.div2 {
  background: mistyrose;
}

.div3 {
  background: coral;
}