title: nth-child targeting all child elements
body: <p>I have the following Demo:</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-css lang-css prettyprint-override"><code>#parent:nth-child(1) {
    color: blue;
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;html&gt;
  
  &lt;body&gt;
    &lt;div id="parent"&gt;
      &lt;div&gt;first child&lt;/div&gt;
      &lt;div&gt;second child&lt;/div&gt;
      &lt;div&gt;third child&lt;/div&gt;
    &lt;/div&gt;
  &lt;/body&gt;
  
  &lt;/html&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>I was expecting only the first child of the parent element to be blue, but instead they all are. Why is this?</p>