"In HTML 4, the only way to create a document outline was with the <h1>–<h6> elements. If you only wanted one root node in your outline, you had to limit yourself to one <h1> in your markup.
But the HTML5 specification defines an algorithm for generating a document outline that incorporates the new semantic elements in HTML5.
The HTML5 algorithm says that an <article> element creates a new section, that is, a new node in the document outline. And in HTML5, each section can have its own <h1> element."
more of Pilgrimさん's Dive into HTML5
or check the outline of your document with the HTML5 Outliner