foo.css

#foo .a + .b {
  background-color: red;
}
#foo .a .b {
  background-color: blue;
}

p1.html

<div id="foo"> 
  <div class="a"> 
    <div class="b"> 
      <p>a .. b</p> 
    </div> 
   </div> 
   <div class="b"> 
     <p>b</p> 
   </div> 
</div> 

What was this supposed to do?