Bug 820508 part 1 - Parser support for <main>. r=smaug.
Bug 820508 part 2 - UA stylesheet for <main>. r=bzbarsky.
Bug 820508 part 3 - Accessibility mapping for <main>. r=dbolter.
Bug 820508 part 4 - Make <main> have the HTMLElement DOM interface. r=smaug.