> "[...] In order to maintain a consistent outline of the page for assistive technologies..."
1.2.1
1.3
1.3.1.1
First you read h1, find the right header. Then h2 with 2 levels of detail, which hits h2 and h3. You miss the sub-section you wanted because it was marked h4 instead of h3.
It's a broken tree structure... that'll cause problems any time somebody wants to do a logical traversal.
[1]: http://www.w3.org/TR/WCAG20-TECHS/H42.html [2]: http://www.w3.org/TR/WCAG20-TECHS/H30.html
As others have mentioned, it's likely flagged as an accessibility issue since having an h4 follow an h2 indicates a broken document structure.
It's also probably included in the tool because things like that usually (though not always) mean that the developer is just using the h4 tag based on how it looks as opposed to actual semantic meaning, and the tool is designed to get you to put more thought into what you're actually building.
It's honestly probably not a big deal but it is an improper use of syntax and could mess with accessibility software in undesirable ways.
As a side note I know almost no one that cares about the semantic structure of an HTML document. It makes me sad. I miss XHTML's rigid structure.
That said, while it's a best practice to keep headings in a sensible order, skipping heading levels is not a WCAG AA violation.
The reason for this is because many screen-readers have keyboard shortcuts to jump from heading to heading. Many users will map out the page with this key-binding. Keeping the headings in a good order is very important.