Ah, so I guess the idea is that this page (with its ~200 comments) would have ~200 iframes added to it?
As what an actual, safe, efficient implementation, of course not.
The point is there could have been a "dynamic HTML" alternate timeline where you could have had interactive pages for 90% of use cases without executing full blown generic computation arbitrary code on the client device.
But worse is better won and now we get insane trackers and cryptominers at every turn because the web can't degrade to exist without JS massaging DOMs.