I've recently started using HTML5 display elements like header, footer, and nav. While reading about the nav element in particular, I found this interesting information in the HTML5 spec:
The nav element is primarily intended for major navigation blocks on a page. However, not all groups of links need to be within a nav element. For example, footers often have a short list of links such as terms of service, home page, and copyright page. In these cases, the footer element alone is sufficient and using a nav element is usually unnecessary.
Why can't we use nav elements in a footer? Many websites actually have substantial navigation menus in their footers, like StackOverflow with a bigger nav menu in its footer than in its header!
If I choose to include a nav element in a footer, will my site fail HTML5 validation? Or is it just a recommendation from W3C?