Really it just parses your HTML and prints it indented so you can see where it's messed up. Sometimes it even fixes it FOR you. Oh that Beautiful Soup.
I've noticed that this handily fixes issues with WordPress WYSIWYG editor (TinyMCE) where it adds hundreds of divs, some of them unmatched. You just copy its generated HTML into this utility and paste the result back into WordPress's HTML window.
Utility Mill is another wonderful Blended Technologies project.
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