600 pixels

This is a box displaying the box model bug problem. Namely, we want it to have a width of 600 pixels with a 100 pixel padding on each side, and a 50 px border. According to the W3C box model, the box should be 600 + 100 + 100 + 50 + 50 = 900 pixels wide. According to Internet Explorer, it should be 600 pixels wide.

900 pixels