I used the Bing webmaster tool's SEO analyzer on my website, and it reported:

There are multiple tags on the page.

It recommends that there should only be one <h1> tag on the page.

The page is a listing of blog posts for a category. So each blog entry is structured like this.

<article>
    <header><h1><a>...</a></h1></header>
    <p>summary...</p>
</article>
<article>
    <header><h1><a>...</a></h1></header>
    <p>summary...</p>
</article>
<article>
    <header><h1><a>...</a></h1></header>
    <p>summary...</p>
</article>
<article>
    <header><h1><a>...</a></h1></header>
    <p>summary...</p>
</article>

How is this invalid? I thought this was the correct way to describe a post in HTML5.