DIVs not abiding by CSS rules

Answer 1

The style definition seems OK.

Do you have any other style defined for div?

Is it working on some browsers and not on others?

You can easily diagnose the problem with web-developer tool or firebug.

Answer 2

try adding this to your page styles:

html, body, form { width:100% }

Answer 3

This kind of problem often is the result of 50% width combined with 1px border. It's 50% + 1px, and therefore more than 50%.