When having miniBB embedded into website with the modified header/footer templates, always avoid <H1>, <H2> and other <H...> tags in their HTML code.

Otherwise, when SEO-auditioning, you could meet various search engines' complaints regarding duplicate heading levels.

The reason is that most important miniBB pages, which are topic pages, already have headings — they mark up topic titles. Some of other pages also include these tags, defining the most important content of the page.

Unless your header defines something unique, structure-oriented, and dynamically inherited from the forum, don't apply the <H1> tag in the header for a visual purpose.

For example, avoid applying it to the forum's or website's title, which is common to all pages. In this case, it won't define the structure. Use <div> or <span> tag, referencing a specific CSS for styling.

For example:

If you have some certain keyword defined as a primary header (<H1>) on every page of your website, it won't boost your SEO. Otherwise: if it appears on each end every page, the forum sends the worst signal to search engines, suggesting all pages as being structurally the same, at the time they are not.

Main keyword(s) should only appear in the <title> tag, and miniBB has long-years development work-out for this, making all topic titles unique, incl. paginated topics, thus optionally combined with a forum name.

I appreciate Steve's information for bringing up this issue.