Feature summary (what you would like to be able to do and where):
Raw headings in markup e.g. <h2>Foo</h2> cannot be easily distinguished from section headings e.g. == Foo ==. This can result in rendering bugs such as T414734: MobileFrontend + Parsoid has different collapsible-section HTML on en.wp main page on mobile.

This is documented on MediaWiki.org but it may be useful given any of the thousands of pages using HTML markup may have similar bugs to have a lint rule specific for this issue

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):

• Headings followed my certain characters intended to be rendered inline might render as blocks.

Benefits (why should this be implemented?):

Clearer behaviour than rely on documentation alone