If your users are using a third-party product to make your product usable, you are doing something wrong.
Activating Safari Reader has a cognitive cost. If your users are activating Safari Reader on your site, this means that the default user experience of your site is so bad that your users first consciously notice that they have trouble reading an article on your site, then remember that they might be able to fix it using Safari Reader, and then actually activate that feature. […]
Don’t fight Safari Reader. Instead, make it obsolete.