<html xmlns="http://www.w3.org/1999/xhtml">
 <style>
  p{ background:red }
  #p1 { background:lime }
  #p2 { background:lime }
  #p3a#p3b { background:lime }
  #p4b#p4a { background:lime }
 </style>
 <p id="p1">This line should have a green background.</p>
 <p xml:id="p2">This line should have a green background.</p>
 <p id="p3a" xml:id="p3b">This line should have a green background.</p>
 <p id="p4a" xml:id="p4b">This line should have a green background.</p>
</html>