E:root

Matches an E element, root of the document - in HTML this is always the HTML-element. (Source: W3C)

HTML:

1. <p>Lorem ipsum dolor sit amet, consectetur</p>
2. <div>Lorem ipsum dolor sit amet, consectetur</div>

CSS:

1. html:root{border:5px solid black}

Output:

The HTML-element should have a 5px black border.