Add the <main> element for the main content

According to the W3C specification, the element <main>, described in
http://www.w3.org/html/wg/drafts/html/master/grouping-content.html
at 4.5.14 section, represents «the main content section of the body of
a document or of an application».

As stated below on the same section, the aria-attributes role="main"
is necessary.

This element was already styled on normalize.css