The content in a web page should be structured usign headings (
<h1>, <h2>, <h3>, etc.).
HeadingsMap is an add-on for Mozilla Firefox that generates an index (or map) of any web document structured with headings. This add-on shows the level of the headings and the errors in the structure (for example, incorrect levels).
A web page without errors:
Headings list in Fangs:
A web page with errors: