<html> <head> <style> div.crossTop { height: 300px; width: 100px; background-color:green; display: -moz-box; display: -khtml-box; display: box; -moz-box-align: center; -khtml-box-align: center; box-align: center; -moz-box-orient: vertical; -khtml-box-orient: vertical; box-orient: vertical; } div.crossBar { height: 100px; width: 300px; background-color:green; margin-top: 100px; } </style> </head> <body> You should see a green cross below (300 pixels wide and 300 pixels tall). This is a test to ensure that boxes don't obey the overconstraint correction (margin-right/left should not be corrected when a box is overconstrained). <div style="padding:100px"> <div class="crossTop"> <div class="crossBar"></div> </div> </div> </body> </html>