Comment by hollerith

Yes, almost any competent developer could define a simple document format that would make it easy to write viewers for all the niche OSes, but he cannot persuade content creators to adopt the simple format because most creators (writers) don't see anything wrong with publishing on the web and don't see anything wrong with relying on intermediaries like Wordpress, Medium and Substack to help them publish on the web. Those intermediaries do not want to switch to a simple document format because the user's being dependent on a mainstream browser helps them to track users and to monetize.

So 99% of the time, a user ends up needing the full complexity of a mainstream browser just to read a static document.