No it's not. It never was. Even a long time ago when websites were static things rendered by the browser once and then left alone there was always a tree of nodes underlying everything. The only thing that's changed in the past three decades is that now when we make websites and web apps we often ship a little JS application that lets the user modify the underlying tree. That's what the web is now and HTML is just what we use to describe the initial state of the tree (and arguably it's the wrong language for that job, but hey ho). Using semantic markup is important for a bunch of things like accessibility, but you can't really expect the initial state of the page to stay the same after it's loaded, and software that renders things from the web has to deal with that.

If you're old enough you might remember that once there were no stylesheets. HTML wasn't just the content and structure, but also the visual description. We used font tags. We used color attributes. We used spacer gifs. We don't do that now because it sucked. We moved forwards. The same is becoming true for HTML. Users demanded interactivity, and we gave them exactly that, and browsers turned in to things that run little applications rather than things that render mark up. The web has grown and evolved and moved on, and thinking of the web as "HTML pages" is just plain out of date.

That's not to say static content isn't still useful. It is. Very useful. But static HTML pages isn't the web, and the single page application genie isn't going back in the bottle.