2.3 Validating Webpages
Web pages can be validated online by visiting http://validator.w3.org and typing the address (URL) of your website. The DTD must be included in all documents. If your code is written correctly a congratulations message will appear after which you can download a logo to include on your site (not necessary), one for HTML, another for CSS. If your site fails you will get an error report immediately listing the errors, after which you can keep tweaking and resubmitting your pages until they are approved.
The validator is useful. It can help locate errors you weren't even aware of. Some browsers try to fix errors in code and show pages as they suspect they should look. With such browsers, some errors may never be spotted at all. However, other browsers might guess differently or not show the page at all. Validate your pages to be sure they will always be shown correctly.