Problem/Motivation
I created this issue as follow-up of #2467827: [META] W3C validation for Drupal Core.
Drupal core added a lot of WAI-ARIA roles to Drupal core (which is a good thing).
A WAI-ARIA role is needed when the markup itself isn't clear. But markup like is superfluous since the HTML elements themselves already explain their role. Having these (duplicate) attributes makes W3C validation fail and we don't want that.
Proposed resolution
Remove attribute Role='main' in tag main.
Files: install-page.html.twig, page.html.twig, maintenance-page.html.twig.
Themes to affect: System Theme, Stable Theme, Classy Theme, Bartik Theme, Seven Theme.