Browsers Are Different
The same page will not look the same on every browser. As a professional designer, you will need to take this into account and possibly change your page to make a compromise.
(you can stop copying here)
Check out the same page in four different browsers (the three columns at the bottom, especially):
Image from (http://archive.cabinetoffice.gov.uk/e-government/resources/handbook/html/2-5.asp).