You can force reader view by entering "about:reader?url=<page URL>" into the address bar, but it can not decide what should be displayed for this page. IIUC thi

You can force reader view by entering "about:reader?url=<page URL>" into the address bar, but it can not decide what should be displayed for this page. IIUC this check runs on all visited pages and the button is only shown when it succeeds.