<html> <head> <style> div { display: -moz-box; display: -khtml-box; display: box; width: 100px; height: 100px; -moz-box-orient: vertical; -khtml-box-orient: vertical; box-orient: vertical; } div.outer { background-color:red; } div.inner { background-color:green; -moz-box-flex: 1; -khtml-box-flex: 1; box-flex: 1; width:100px; height:10px; display: block; } </style> </head> <body> <p>You should see a single 100x100 green square below. If you see any red, then the test has failed.</p> <div class="outer"> <div class="inner"></div> </div> </body>