Validation Resources - Web Center
An HTML validator checks the correctness of your document against the declared DOCTYPE of the document. All (X)HTML pages should be validated. More about DOCTYPEs in XHTML and HTML.
Validate your web pages to ensure their compliance with web standards.
Style Sheet (CSS) Validators
Validate your style sheets to ensure their compliance with web standards. You can validate by entering the URL of your external style sheet, or by copying and pasting your style sheet code into a form box, or by uploading your external style sheet file from your local hard drive.
Check your pages to ensure that the links are working. You may find that some links that are reported as broken actually do work correctly. This occurs usually with links using "https:" (instead of http:) formatted URLs which indicates a secure connection is being used.