title: Best way to feature detect :focus-within pseudo-class
body: <p>I can't find any syntax using @supports that works with pseudo-class, except for <code>not</code> which is very specific to that pseudo-class.</p>
  
  <p>What is the best way to detect if a browser supports focus-within, as I don't want to write a lot of browser hacks that will change in a few versions? I'm using Node and npm, but don't want to import a whole package for it (usually not the best way to handle things).</p>
  
  <p>Trying to read the browser version string or use today's browser parsing hacks isn't a good idea because it will be difficult to maintain when browsers do start supporting a feature, so I'm looking for a feature detection method besides @supports. Is there a way to determine this in javascript?</p>