No, not strictly, but there are some contextual semantics that are implied by embedding the header
under elements other than the body. Depending on how you structure the document, you may run into unexpected behaviours. See below.
HTML5, as far as I can tell, is a DTD-less document type. Which does not apply strict rules as it pertains to document structure validation. According to the W3C document on HTML5
the:
... header element represents introductory content for its nearest ancestor sectioning content or sectioning root element. A header typically contains a group of introductory or navigational aids. [1]
Basically, what this seems to indicate is that, if the header
element is directly under the body
of the document, it applies to the page as a whole, but if contained under another element which identifies a section the header
applies more specifically to the nearest ancestor.
Bear in mind though that a <div>
is not a sectioning element as pointed out by Tomasz Zielinski. Originally I had thought it was. If you want your header to apply only to a specific section of the page, it is best to use one of the defined sectioning elements to encompass the header, as defined here: http://www.w3.org/TR/html5/sections.html
E.g.
<div>
<section>
<header>...</header>
</section>
</div>