Validating a website is the process of ensuring that the pages on the website conform to the norms or standards defined by various organizations. Validation is important, and will ensure that your web pages are interpreted in the same way (the way you want it) by various machines, such as search engines, as well as users and visitors to your webpage.
Conforming to standards and regulations is one of the many ways you can make your website universally understood. Make sure your codes and styles validate across the board. That means they have to meet the "strict" standards set by the W3C Organization and pass a variety of validations for CSS and XHTML.
Not all validators check for the same things. Some only check CSS, others XHTML, and others for accessibility. If you are sincere in presenting standardized pages to the public, test them with several validators. The World Wide Web Consortium sets the standards and also hosts a variety of web page validators.
In simple terms, validation ensures that your website complies with the standards accepted by most web designers. That also means that it will be accessible to more people, across more web browsers and operation systems. Having an accessible website is also regarded as good web design practice. As you can see, having a validated website has its definite plus points.
Validators detect problems in your web page and style sheet. It could be a tag that was opened and never closed. It could be a misspelled piece of code or forgotten element the tag or style requires to work properly. You become a detective, hunting and solving the little problems occurring in your web page. The resources and articles on validation below will help you learn more about validating your web page.
Validating your WordPress site means more than just checking the front page for errors. With the modular Themes and template files in WordPress, while you may fix all the errors associated with the index.php and sidebar.php when viewing your front page, errors may still lie within any other template files such as single.php, page.php, archives.php, or category.php. Validate these page views as well to make sure you cover all the template files.
Validation errors aren't limited to your template files. They can also happen inside of a post. When you are writing a post and using HTML, WordPress might not recognize the HTML and convert it to a character entity, or you may have entered it wrong. If you have a page with a lot of HTML coding, validate it to make sure you have it all correct. And occasionally check random posts to make sure everything is still okay from time to time as part of your general housekeeping.
Validation doesn't just mean putting your pages through some web driven testers. It also means test-driving it with friends, relatives, co-workers, and strangers. Everyone has a different system and way of working, so ask for others to test-drive your styles or themes before you make them public.
The Your WordPress section in the WordPress Forums is dedicated to helping WordPress users in getting feedback about their sites. WordPress volunteers will do that for you for free. Be sure and read the WordPress Site Reviews Guidelines. Help is also at hand there to solve your problems, and reviewers might just go 'ooooh' and 'ahhhhhhh' over your website, which is very gratifying, we hear :).
To help you validate your WordPress site, here is a quick checklist: