I think the article provided good insight on this: "I wonder if the reason this approach makes more sense now is that we’re doing more generation of HTML than we were in 2003. In my tiny example, this approach to CSS actually doesn’t introduce that much duplication into my site, because all of the HTML is generated by Hugo templates, so most styles only end up being specified once anyway."

So you write your button one time in a reusable button component, then reference the component and avoid writing the same 20 styles again. I think this has benefits as it keeps all the information for a single component in the same location, making it easy to understand and edit the component from a single location.