What is a good HTML5 Outliner?

I've been reading Dive Into HTML5, and Mark mentions that:

"You can test your own pages in the HTML5 Outliner to ensure that you’re using the heading elements properly."

I've tried this HTML5 Outliner and I know that it is functional from outlining other HTML5 pages I've found on the Internet.

The trouble with it is that this doesn't work on all pages (mainly the ones I have written for a Wordpress theme), and the error messages are cryptic; they seem to refer to the problem element in question (<section> for example), but don't do much to specify which element it is talking about on the page; i.e. specifying a line number, like the validator does.

I've found some other programs that are capable of reading html5 outlines, but I'm concerned that they may be early implementations that may be out of date for the current HTML5 spec.

So I was wondering if there was another program, website, etc, with more concise error messages that I could test my pages to ensure that I (a beginner in HTML5) is creating a document outline that could be properly read by tools capable of reading HTML5 document outlines.

The W3C HTML Checker at https://validator.w3.org/nu/ has option to generate an outline: