Anyone with minimal knowledge could start sharing content. If they had to pass any kind of validation, many would not have bothered. Yes it was a mess, but it helped us get enough information to reach critical levels. I would rather have a web with messy HTML than FTP servers filled with Word documents or a proprietary AOL / MSN
As a general principle, pushing denormalized data as far to the edges of the system as possible minimizes complexity of the system as a whole.
The thing about the web is that it was for amateurs. Things like separation of content and style are great for engineers creating larger-scale maintainable sites, but they're absolutely not helpful for new users. Being able to take a text file and add a couple of tags around some text you want to emphasise is a positive advantage for such individuals. It rewards small experiments. Understanding CSS is, by contrast, quite the undertaking.
Hypertext had been around for quite some time before the web, and there's still some academics I know who seem to scratch their heads at the success of such an simplistic, impure form. To me, its simplicity is a reason for its success, while projects like Xanadu make a fundamental error in not considering enough the importance of barrier to entry.
When asked, "How could you possibly have done the first interactive graphics program, the first non-procedural programming language, the first object oriented software system, all in one year?" Ivan replied: "Well, I didn't know it was hard."