title: Conditional CSS: if sibling's child div is present, then
body: <p>In the html fragment below, I want the "main" div to have a background image only if "menu" div is not present in the markup. Is this possible?</p>
  
  <pre><code>&lt;div class="header"&gt;
      &lt;div class="siteTitle"&gt;site title&lt;/div&gt;
      &lt;div class="tagline"&gt;site tagline&lt;/div&gt;
      &lt;div class='menu'&gt;&lt;/div&gt;
  &lt;/div&gt;
  &lt;div class="main"&gt;&lt;/div&gt;
  </code></pre>