Great websites need to be clean and clear. However, sometimes even the best testers can overlook simple coding errors. A free tool, the W3C Validator, catches these. The W3C Validator automatically compares web pages and web sites that use the HTML5 languages to known good specifications. The W3C Validator reports errors and warnings. The W3C Validator was used on 10/8/2013 to check some common websites: cnn.com, google.com, and facebook.com. The validation report provided feedback on over 20 lines of code, noting some common errors. This paper describes how to use the W3C Validator to catch what human eyes tends to miss.