Hmm... I don't think the comments can do html. I'll check.

I've gotten it to consistently retain the class="spoiler" thing in the version stored in the database, but I think it's getting stripped when it builds the HTML page. Alas.

It's set. The software seems to be retaining the <p> but stripping the class="spoiler", unfortunately.

Hmmm... maybe you're using a plugin that's killing it? Or perhaps sanitize is? Really, just guessing at this point.