Thanks - good links.
One of the possible problems: text/data being added or hidden by the stylesheet doesn't seem so problematic to
me. (On first glance anyway - I'll mull over it!!) The idea would be that every topic or discussion
group would be owned by one person, who would set
the stylesheet. If they want to add "BIG LIE" (as per
the link's example) that's their business.
On the other hand, it does seem like a fraught business,
maybe I will go with accepting CSS sheets in a form and
ratifying them centrally before releasing them to the
general public. What a pain!
(By the way, TheHobbit pointed out that my
link to an external site above was wrong: thanks!
seems to work.