title: adding an element only between two elements of given class (with css)
body: <p>I'd like to know if it's possibile insert a new element between two elements of a given class. I'like to use only css, for exammpe:</p>
  
  <pre><code>&lt;div class="a"&gt;
  &lt;div class="a"&gt;
  
  &lt;div class="b"&gt;
  &lt;div class="b"&gt;
  &lt;div class="b"&gt;
  &lt;div class="a"&gt;
  
  &lt;div class="b"&gt;
  &lt;div class="a"&gt;
  
  &lt;div class="b"&gt;
  &lt;div class="a"&gt;
  </code></pre>
  
  <p>I'd like to insert a new element between "a" and "b" (but not vice-versa):</p>
  
  <pre><code>&lt;div class="a"&gt;
  &lt;div class="a"&gt;
  &lt;blank-line&gt;
  
  &lt;div class="b"&gt;
  &lt;div class="b"&gt;
  &lt;div class="b"&gt;
  &lt;div class="a"&gt;
  &lt;blank-line&gt;
  
  &lt;div class="b"&gt;
  &lt;div class="a"&gt;
  &lt;blank-line&gt;
  
  &lt;div class="b"&gt;
  &lt;div class="a"&gt;
  </code></pre>