It would be nice on some pages or content to see the site in reader mode (native to the browser)

This isn't really something websites "support". 

This is an interesting read: https://www.ctrl.blog/entry/browser-reading-mode-parsers . The short version is, different "reading mode" parsers behave differently, most have not published any guidelines for people to follow, and they really just do their best to guess what is content and what is meta data in most cases.