I quite agree. HTML/CSS/JavaScript is really flexible for sure, but it's because humans happen to be capable of parsing, and extracting information from, what would be chaos for machines. Is this flexibility really necessary? In other words: How many people jump to the Reader View button when reading a long article? What is the success rate of that functionality? Hasn't anyone realized this functionality wouldn't have been necessary if there were limited flexibility available to web contents?

Imposing structures on webpages as suggested would incur limitations, but it's a necessitated consequence of having content with machine-readable semantics. Think of how we encode characters -- we don't encode them by vector graphics of each character or two-dimensional array of pixels. But rather, we encode characters by concise code points. Yes, we lost flexibility by that. In plain text files we cannot represent different fonts because characters are simply represented with a sequence of code points. But don't the advantages outweigh the disadvantages by a large margin? We can read them in whatever font we like, we can `diff` text files... the list goes on and on. It's unfortunate that web ended up as a huge mess which machines can never reliably comprehend.