The Web Design Group wrote this about the structure of HTML 3.2 documents back in 1996:
Writing a structured document does not mean that you are writing in a straitjacket. It only means you have to lay out the document in advance. It also means the document becomes easier to read, maintain and extend. While this may not seem too important if you just want a homepage, when you have a whole site to maintain, well-structured documents make life a lot easier!
It’s interesting to read all these years later. We’ve come a long way with standards compliance in browsers, but document structure and maintainability haven’t kept pace. If anything the industry has gone backwards, with additional layers of brittle abstraction, and code so nested I could sell it to Horse Brand.
Try it out: view the HTML source of almost any website you go to in 2020. It may be standards compliant in the strictest sense, but would you call it well structured? Would you call it maintainable? Would you even call it understandable? Then extrapolate that to the CMS, and the web server, and the networks.