tldr; anything beyond unstyled, purely textual HTML in a single column, without plugins (and
definitely without javascript) is objectively wrong and bad and people shouldn't be allowed to do it. I'm assuming images are fine but I wouldn't be surprised if not.
The last paragraph in this manifesto is the author pre-complaining about every possible change that will be made in HTML in the future. How much more "hipster yells at clouds" can you get?
As I too am a Person On The Internet, allow me to retort:
HTML was never entirely about displaying plain text, but content. That that content is primarily text doesn't imply it must be primarily text. That train left the station when img tags became a thing.
The ability to embed plugins was a breakthrough, not a mistake. Plugins are the reason video on the web is possible, and video on the web is revolutionary for sharing culture and expression. Flash and Java applets, for all their faults, allowed applications to be accessible by URL, and thanks to HTML5/Canvas and tools like asm.js and whatever-to-js transpilers, we're on the verge of having the web become a network of hyperlinked applications. When you think about the ramifications of that, it's a really big deal.
To want to forbid the web from even reaching the complexity and richness of HTML 1.0 is, frankly, asinine and presumptuous. Notwithstanding that, yes, simple text-based sites have a place and, yes, most uses of javascript to display simple text are kind of pointless and absurd, the author is wrong to want to limit the world's creativity and the web's expressive potential to the boundaries of his own personal elitist prejudice.