The Little Black Book of Online Business: 1001 Insider Resources Every Business Owner Needs

75.3. HTML VALIDATION

This service will validate the code that makes up your site, informing you of any detected errors:

http://www.w3schools.com/site/site_validate.asp

To verify that your site is viewable in different kinds of browsers, use http://browsershots.org—it will create snapshots of your site as viewed from various browsers:

http://browsershots.org/