Yes, you are correct. It's impossible to validate HTML from DOM traversal alone because the browser has already parsed it and forgiven any errors it was able to

Yes, you are correct. It's impossible to validate HTML from DOM traversal alone because the browser has already parsed it and forgiven any errors it was able to. (I didn't choose this title and agree that it's a little misleading.) Technically, you could validate the markup if you did an AJAX request for the current URL and parsed the result yourself; however, if your app is generated dynamically and rendered primarily client side, you'd be missing a lot of the alterations done by your JavaScript in that validation. This is why I think both tools have their place. I try address this issue in my blog post introducing HTML Inspector under the section "How is HTML Inspector different than the W3C Validator?" http://philipwalton.com/articles/introducing-html-inspector/ http://philipwalton.com/articles/introducing-html-inspector/