Problem with that is that the default browser styling is extremely ugly and the ability for custom style sheets was removed from the browser GUI many years ago. ReaderMode and Addons can help, but as long as the default is essentially broken and unsupported that whole approach remains a dead end.

On top of that come issues like the lack of pagination support in browsers, which make long document impossible to read and practically require to add custom UI inside the website itself.

ePub works much better, with readers giving control over line spacing, font size, pagination and proper markup for TOC and other metadata, but despite ePub being based on xHTML, browsers have ignored it (only old Edge supported it for a little while).