What Propery(a trait) a page must have, for displaying reader view Icon on that page?

For example firefox show reader view icon for all Wikipedia webpages…
my question is , what property must add to a page( HTML,CSS,Meta tag,Article Tag,… etc ) , to allow firefox detect that page and show a reader view icon on it?

Very Thanks

The code is in Readability.js. It checks for structure, names of css classes, size of text blocks if I remember correct, …


Takes some time for understanding from source code , but here from github examples it’s easy:

and tests https://github.com/mozilla/readability/tree/master/test