Reader mode on Royal Road stories shows CSS-hidden messages

Originally filed at: https://bugzilla.mozilla.org/show_bug.cgi?id=1987827

Steps to reproduce:

• Open any story chapter on Royal Road; for instance, https://www.royalroad.com/fiction/118891/new-life-as-a-max-level-archmage/chapter/2324263/1-vivisari
• Turn on reader mode
• Somewhere in the page, buried in the chapter, you'll see a message like "If you come across this story on Amazon, it's taken without permission from the author. Report it." or "Help support creative writers by finding and reading their stories on the original site." or similar. There are various rotating messages. In that example URL, it appears right after the text "No response".

Royal Road embeds these messages in stories to fool simplistic scrapers. They all have CSS classes (with randomly generated class names) that are styled with a style element like this:

        <style>
            .cjNkODUzODExNzk4OTRiODhiMTU2ZDY0MDcwNWJiMWU5{
                display: none;
                speak: never;
            }
        </style>

It'd be helpful if Firefox's reader mode hid these. This doesn't require loading a separate CSS file; the styling is on the same HTML page.

Start by reproducing the Royal Road chapter in reader mode and inspect how its same-page style element and CSS-hidden messages are handled. Done means those messages no longer appear in reader mode while the visible chapter content remains intact.