I recently worked on implementing readability in my Hacker News client app and here's the resources I found most useful. Basically, if once designs their site following the correct structure, then it works correctly for both reader mode as well as accessibility. These are "standards" which sites are supposed to follow but they don't unfortunately:

A very long read but this covers everything:

> How to Section Your HTML

https://css-tricks.com/how-to-section-your-html/

These are specifically important for accessibility users.

Another short read:

> HTML5 sectioning elements, headings, and document outlines

https://www.456bereastreet.com/archive/201103/html5_sectioni...

And you can use Markup Validation Service to validate and get tips:

https://validator.w3.org/#validate_by_input+with_options

Check out the excellent answer by Christian Kohlschütter on how readability works:

https://stackoverflow.com/questions/3652657/what-algorithm-d...