This page discusses two types of validation:
XHTML and CSS validation
You can validate XHTML and CSS to ensure that your code is free from errors and potential bugs, that it adheres to best practices, that it is able to be tested properly, and that it provides the foundation upon which you can build an accessible Drupal theme.
The W3C provides the following validation services for both markup and CSS:
Most Drupal themes use the XHTML Strict doctype and aspire to be valid, however not all achieve this. If you are modifying an existing theme you should first validate the theme to identify any problems.
The most common CSS profile to validate against is CSS2.1, however many themes now use CSS3.
In order to quickly identify accessibility issues with your theme you can use one of the following accessibility validation services or tools. Some of them are browser toolbars and some are external services similar to the W3C validators.
- Total Validator
- Functional Accessibility Evaluator (FAE)
- Web Accessibility Evaluation Tool (WAVE)
- Juicy Studio Accessibility Toolbar
- Web Accessibility Toolbar [For Opera]
The WAI has a more extensive list of web accessibility evaluation tools.