@Alex: Oops, missing in style.css, therefore the browser's default definition for <h1> (within the "content-class") is applied...

Yes, class="postingheadline" is needless (.ct-posting h1 is applied).

only one <h1> should be used within a page

In my opinion there can be one <h1> for the website headline (like here "my little forum") and another one for the article/document/posting headline - and even more if there are several articles with the same relevance on one page.
But you're right: an opened complete thread is a hierarchical structure so the initial posting headline should be marked up with <h1>, the headlines of the replies with <h2>.

Alex