Add a space between the heading selectors
Add the <main> element to the list of HTML5 selectors
Merge pull request #278 from jayj/jayj-underscores-main-element

Add the `<main>` element to the list of HTML5 selectors.
Merge pull request #279 from jayj/jayj-underscores-heading-spacing

Add spaces between heading selectors.
Merge branch 'master' into ja