This issue has been solved. Thank you, @stetim94!
I am on 23. Pseudo Selectors, here's the css:
I have tried
Yet, none of them work as intended.
The error returned is as shown in the title.
Any help would be of up most appreciation.
The HTML as requested
<link type="text/css" rel="stylesheet" href="stylesheet.css"/>
<p id="serious">Paragraph 2</p>