The NetBeans HTML5 editor is pretty amazing, working on an extensive screencast on that right now, to be published soon. One thing missing is HTML Tidy integration, until now:
As you can see, in this particular file, HTML Tidy finds 6 times more problems (OK, some of them maybe false negatives) than the standard NetBeans HTML hint infrastructure does.
You can also run the scanner across the whole project or all projects. Only HTML files will be scanned by HTML Tidy (via JTidy) and you can click on items in the window above to jump to the line.
Future enhancements will include error annotations and hint integration, some of which has already been addressed in this blog over the years.
Download it from here: http://plugins.netbeans.org/plugin/51066/?show=true
Sources are here. Contributions more than welcome: