XHTML Validator

Submit your Website and check for Markup Validity, XHTML 1.0 Transitional on the below link.

http://validator.w3.org/

And give the website URL and check out the xhtml errors if any.

CSS Validation

Verify Casceding Style Sheet(CSS) used in your website to display your valuable content before going in production.
Follow the given link and enter the URL of your website and check out the response.
http://jigsaw.w3.org/css-validator/