This person needs to go look at some Geocities archives. People were trying to create cool-looking pages as soon as we had the IMG tag. People were using tables to organize a bunch of images in neat ways. People were doing weird little hypertext art things.
They were not using the technology in the "right" way. Nobody gave a shit about the right way. It took something like twenty fucking years for CSS to make it easy to vertically center shit the "right" way after persuading everyone that using tables for anything but tabular data was "wrong". If we waved a magic wand and suddenly everyone just had this theoretical Markdown-centric browser? People would immediately start looking for clever ways to abuse edge cases of its implementation to make their pages pretty. And people would start making enhancement requests, some of which the browser makers would inevitably implement, and... eventually we're right back where we are now.
I am not a fan of the modern corporate-dominated web, neither am I a fan of the modern world where more and more apps are horrible kludges of JS and HTML that have absolutely no care about the UI conventions of their host platforms, and are a couple orders of magnitude more resource-hungry than their equivalents that use native widgets and compiled languages. (Concrete example: Slack's 440 megs on disc, Discord's 370; Ripcord, a native app that talks to them both, is 40.) But thinking people will happily go back to a world with little to no control of the presentation layer after decades of struggle for more of that is a pipe dream. We all quit using Gopher the moment we had a web browser on our systems.