Originally Posted by duckduckgoose

You can submit your websites through a CSS/HTML validator, and that will check them to make sure they are compliant code. This will make sure no major bugs are present that would affect rendering on a given compliant browser.

Firefox itself has some built-in tools to help this process. The "Web Developer" extension often comes built in to Firefox. It is really helpful to anyone developing websites. Nice big toolbar along the top with options and features for web dev. Choose Tools, then Validate CSS or Validate HTML.

Check it out -- you probably already have it installed