Отступ у ребенка body

.parent { 
  margin-top: 100px; 
  width: 100%; 
  height: 100px; 
  background: #000; 
  overflow: hidden; 
} 
 
body { 
  height: 100%; 
  width: 100%; 
  overflow: hidden; 
} 
 
body:after { 
  content: ""; 
  display: block; 
  width: 100%; 
  clear: both; 
  height: 100%; 
} 
 
.test { 
  margin-top: 30px; 
  background: blue; 
  height: 5px; 
  width: 100%; 
}

<div class="parent"> 
  <div class="test"></div> 
</div>