title: How could I add a header to a HTML page via CSS?
body: <p>There is a script that uses Pandoc to produce a HTML representation of an input Markdown file. Assuming that the only control over the HTML representation I have is the specification of a single CSS file, how could I add a site title/header to the HTML?</p>
  
  <p>For example, let's say I wanted to add some very simple heading a bit like the "ATLAS Briefings" bar at the top here, how would I do it in CSS, not in the HTML of the page?:</p>
  
  <p><img src="https://i.stack.imgur.com/dVeVF.png" alt=""></p>