Allowing main element as well as main role

[nicholasgriffintn]

Allow <main> as well as <element role="main'> as this is even more valid.
https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Roles/Main_role

As discussed / raised in #336

[votemike]

It seems like that is because

is an HTML 5 element that IE11 doesn't understand (and therefore doesn't apply the role to).
IE11 is not unsupported and so I see no need to enforce this rule.