A CodePen by fff.

index.html

<div class="blue outer">
  <div class="yellow">fff</div>
<div class="purple margin10">FFF</div>
<div class="yellow padding10">fff</div>
</div>
<div class="outer purple">yyy</div>

style.css

div{
  border:  #000 0px solid
}
.blue{
  background:blue;
}
.outer{
    width:90%;
}
.yellow{
  background:yellow;
}
.purple{
  background:purple;
}
.margin10{
  margin: 10px;
}
.padding10{
   padding: 10px;
}