Added `main` element

The `main` element was [formally added to the HTML5 spec](http://html5doctor.com/the-main-element/) earlier this year and should be reset to `display: block`. `main` element added to both `"block"` and `"html5-block"`
Tests for 32b3a49f55b06691dcdd9f135ba2a95184a17ca9

Closes #1445.