Check Your Website in IE and Firefox
Many webmasters make the mistake of only viewing their site on one browser before setting it up on the server. This may backfire in more ways than one.
To make sure you do not make the same mistake, check your website in as many browsers as possible. The most important depends on your site logs (if you already have the site up that is), however the most common browsers are IE 6.0, IE 5.5, Mozilla Firefox and Opera and if possible you may want to view the sites on a Macintosh as well.
The differences between the browsers may be as small or as large as you make it. However, with the correct CSS and valid markup the task should be easier than expected.
Some differences between the browsers include the box model, CSS hover and some margins along with font issues. So it is important to check your site on as many browsers as possible or your site may be broken to 15-20% of your viewers.