AssistiveMML compatibility with "browser reading mode"

Many browsers have "reading modes" these days and a user pointed out problems with that because these modes change CSS (a lot) and can make the hidden MML visible.

The user also pointed me to https://bugzilla.mozilla.org/show_bug.cgi?id=1144822 which lists several class names that Firefox's reading mode finds "acceptable" (based on framework defaults).

It would make sense to add a "commonly used" class name to avoid duplicate visual content.