According to HTML5Doctor.com and other HTML5 introductory sites, the header element should contain a h1-h6 tag plus other navigational or introductory content. However, the traditional 'header' on most websites consists of just a logo and some navigational elements.

A lot of major sites including Facebook and Twitter use a h1 tag for their logo, which seems illogical to me, since the logo is not the most important or most informative element on the page.

A h1 tag appears in the content section of 95% of websites, not the header section. So why are we instructed to include a h tag in the header? If we must, why don't Facebook and Twitter use a h6 tag instead?