TIL about XHTML + ab. CSS escapes

screenshot of source code:

<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml" lang="en">
  <head>
    <title>self-closing strong element</title>
		<style>
			html { font: 2em/1.2 Calibri, Helvetica Neue, sans-serif }
		</style>
  </head>
  <body>
		<p lang="la">Ad astra per <strong/> aspera</p>
		<p>To the stars through hardship</p>
  </body>
</html>