There are several useful tools for testing (or validating) your XHTML and CSS for errors. This process is often called "validation" because your site's code is checked to see if it conforms to the www.w3.org standards.
XHTML validation
XHTML mark-up validation at http://validator.w3.org/
CSS validation
CSS validation service at http://jigsaw.w3.org/css-validator/
Other validation services
Click here for a comprehensive list of validation links.
Screen resolution & favelets
You can use favelets (JavaScripts in bookmark/favourites form which re-size your browser window) to test how your pages will look at various resolutions.
Click to visit http://www.tantek.com/favelets/