Edit in JSFiddle

header{
   h1{
      font-size: 25px;
      color:#000;
   }
   
   p{
      font-size: 15px;
      color:#3e3e3e;
   }

   .container{
      h2{
         font-size: 20px;
         color:#5b5b5b;
      }
      
   }
}
<header>
    <h1>This is Heading</h1>
    <p>This is paragraph</p>
    <div class="container">
        <h2>This is sub heading</h2>
    </div>
</header>