title: How to check if multiple elements exists
body: <p>Given this html:</p>
  
  <pre><code>&lt;div id="wrapper"&gt;
       &lt;div id="header-holder"&gt;
            &lt;div class="header"&gt;header 1&lt;/div&gt;
            &lt;div class="header"&gt;header 2&lt;/div&gt;
       &lt;/div&gt;
       &lt;div id="project"&gt;project data&lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>I want to apply a style to element in <code>.header</code> only if <code>#project</code> exists. I'd like to do this with css. Is this possible?</p>