title: Responsive design. Setting min-width in em
body: <p>First example :</p>
  
  <pre><code>@media only screen and (min-width: 400px) {
  body { background-color: blue; }
  }
  </code></pre>
  
  <p>Rule will work with extensions more screen 400px.</p>
  
  <p>Today met a different style to set rules :</p>
  
  <pre><code>@media only screen and (min-width: 40em) {
  body { background-color: blue; }
  }
  </code></pre>
  
  <p>What is the idea of using 40em ...? It's not even a fixed width. Under what conditions will apply this rule?</p>
  
  <p>When to use this rule?</p>