While Googling for the W3C HTML validator page to add it as a bookmark, I found that there is a plugin for Firefox that will validate the code right in the browser. It has the option to run the same backend validator engine as the service at W3C (the SGML parser), to run a more thorough validation engine (the Tidy parser; also developed by the W3C) or to run them serially (i.e., the W3C engine and then the more thorough one if no errors are found.) The Tidy parser can also do accessibility checks and suggest fixes, among other things. The plugin can be found at https://addons.mozilla.org/en-US/firefox/addon/html-validator/. Keep in mind that you will get more errors with the Tidy parser. Playing with some “real” sites is interesting. Google yields 60 errors. Amazon came up with either 160 or 1566 depending on how the results are read. (I think it’s different reading a frame vs the entire page.) Reddit has 45. It looks like it might have some issues with HTML5, but you simply have to “view source” in the plugin menu. It also has a direct link to the online HTML validator (sending the page) as well as the CSS validator.