Inténtalo tú mismo - W3.CSS responsive_half

<!DOCTYPE html>
<html>
<title>W3.CSS</title>
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="/lib/w3.css">
<body>

<div class="w3-container">
<h2>Mobile First Responsiveness</h2>
<p class="w3-large">Try to resize the window!</p>
</div>

<div class="w3-row">
  <div class="w3-half w3-container w3-green">
    <h2>w3-half</h2>
    <div class="w3-row">
      <div class="w3-half w3-container w3-red">
        <h2>w3-half</h2>  
        <p>This is a paragraph.</p>
      </div>
      <div class="w3-half w3-container w3-yellow">
        <h2>w3-half</h2>  
        <p>This is a paragraph.</p>
      </div>
    </div>
  </div>
  <div class="w3-half w3-container w3-blue">
    <h2>w3-half</h2>
    <div class="w3-row">
      <div class="w3-half w3-container w3-red">
        <h2>w3-half</h2>  
        <p>This is a paragraph.</p>
      </div>
      <div class="w3-half w3-container w3-yellow">
        <h2>w3-half</h2>  
        <p>This is a paragraph.</p>
      </div>
    </div>
  </div>
</div>

</body>
</html>