Let's define "properly." I would say when the html and css each do their own jobs with nothing added, and your css doesn't lock you in to proprietary classes -- where html is semantic and handles accessibility without help. I've found pico.css a useful example because it doesn't require the use of classes at all. Jason wasn't thrilled with its css size, but it works well and its use of custom properties makes it very easy to customize colors and sizes, and achieve consistent page designs.