Taken from this question,
And this site,
Why would they place the web site within a form element.
<form id="RunwayMasterForm" action="/en-us/" method="post">
Looking at the markup structure, the form element is a direct child of the body and all other elements are contained within the form element.
Why would they do that?
In the original form of Microsoft’s ASP.Net (Web Forms), all pages generated markup that included a single form tag wrapping everything.
My guess is that either the developer originally created the page as part of an ASP.Net WebForms “solution”, and since migrated away from that framework without removing the form tag, or just developed a mis-understanding of this as being a kind of standard, though working with such solutions over a period of time.
udpate looking at the link (Windows Azure) in the SO question you referenced, this is a current MS page — yes, indeed, this is WebForms in action.