Whenever I test a new extension, one of the first things I test is validation. Using the W3C validation service I can check if the extension outputs validating code.
The Markup Validation Service is a validator by the World Wide Web Consortium (W3C) that allows Internet users to check HTML documents for conformance to HTML or XHTML standards. It also provides a quick method for web page authors to check their posted pages for mark-up errors.
Running your site through a check like this is valuable and and easy way to check if you have made any mistake in your coding.
And, as I mentioned, it is a really good way of knowing if the brand new extensions you installed in your Joomla site will produce validation errors on your page.