Edit in JSFiddle


              
<!DOCTYPE html>
<html>
<body>
  <h1>Top</h1>
  <section id="main">
    <h1>Main Level</h1>
    <section id="article">
      <h1>Second Level</h1>
      <p>Yup</p>
    </section>
  </section>
</body>
</html>
* { font-size: 14px;}
#main h1 /* descendant */
{
  color: Red;
}
#main > h1 /* child */
{
  font-size: 24px;
}
h1 + p /* adjacent siblings */
{
  border-bottom: black 2px solid;
}