HTML's Most Underrated Feature is Testability

I’m an old programmer as these things go. One of the best things I provide is perspective. You’re probably thinking that means I’m going to reminisce about the good old days. But I think the good old days are hilariously overrated.
HTML-based GUIs get a lot of flack for being a cheap-to-build imitation of native-app. This is basically true. It’s a cheap imitation, so we can build (and test!) a lot more with a small team, for cheap. In return, they burn a lot more CPU and memory, which is sometimes okay and sometimes not. I’m building something like that right now, so obviously I think it has good points.
But I think the biggest advantage of HTML-based GUIs isn’t usually mentioned: testability.