Validating your Joomla! template's XHTML can be a time-saver when it comes to debugging problems with your design's display in different browsers. Although pre-existing Joomla! templates may well contain errors in their HTML, if you're creating your own template you may find it useful to find errors in your Joomla! template.
Rather than visiting the W3C validator directly, some browsers provide tools to allow you to validate pages more conveniently. The W3C acts as the primary organization in creating standards for the website, such as CSS, XHTML, and HTML.
Opera (http://www.opera.com) has an option to validate the current page when you right-click:
The Web Developer add-on (https://addons.mozilla.org/en-US/firefox/addon/60) provides a similar feature for Firefox: