Validating css and xhtml
Each tool provides numerous options so that you can set it to best suit your needs.
CSS Validator: Adds an easy-to-use link to the W3C CSS validator that you can access via a right-click context menu or from under the Tools menu.
Page Validator: Either by opening the right-click context menu, or going up to tools and selecting “Validate Page”, Page Validator opens a separate tab and shows you the results using the the W3C’s online validation tool.
RSS Validator: RSS Validator allows you to easily check the validity of an RSS feed by using the right-click context menu or by making the choice from the tools menu.
You can choose which version of HTML for it to use, whether to display the source code, use a “dirty” parser, and there are a couple of other options.
It is also available as a Firefox plugin, which adds the service directly to your browser.
xhtml-css.com: gives any site a quick once-over, validating both the HTML and CSS in one go.