Add HTML5 `main` tag

This adds rudimentary support for the HTML5 `main` tag, but there might be additional logic desired for this qualification:

```Note: There must not be more than one <main> element in a document, and it must not be a descendent of an <article>, <aside>, <footer>, <header>, or <nav> element.```

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/main