chromium/src/third_party/blink/web_tests/css3/selectors3/xml/css3-modsel-d1b.css

   #test1 { background: red; display: block; padding: 1em; margin: 1em; }
   #test1:empty { background: lime; }
   #test2 { background: lime; display: block; padding: 1em; margin: 1em; }
   #test2:empty { background: red; }