I recently worked on implementing readability in my app and here's the resources I found most useful:

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...