"Iirc approved html is fixed up on the fly."

I don't have access to the PM code so can't comment on this directly. I'm not even sure what "fixed up on the fly" actually means (seems a bit vague).

"Each reader can choose how spoiler blocks will be rendered."

This is a verbatim quote from "Perl Monks Approved HTML tags" (footnote 4) except the word "choose" (on that page) is a link to Display Settings.

In Display Settings, the Special Tag Options table has "Render <spoiler> tags as" with a selection list giving six options.