We have already seen standalone HTML validator application to check for errors in HTML files. If love browsing and do it all in Firefox then you can use HTML Validator Firefox plugin. It shows number of errors and warning as an icon at the bottom right part of the Firefox window. You can view HTML source code to see more error details.
You can easily toggle between original HTML and cleaned up HTML. Errors are highlighted which can be easily edited. You can add indents to element content or set tags and attributes to uppercase. Download HTML Validator Firefox extension to check for HTML goofups within Firefox. It is easy and nicely integrated with Firefox browser.