I honestly have no idea what you’re talking about. If you’re directly responding to my comment saying “All the more reason not to use it”, I guess one of us misunderstood the other. In seemed to me like you were saying that the user-select CSS is experimental, so it seemed like you were making my argument for me: to not use it. Considering that Discourse is using this experimental CSS right now (from looking at the HTML elements used for spoiler text), that was the only conclusion I could come to.
Now to your actual response. What about my feature request to allow you to quote spoiler text would result in the removal of another feature? And what is the well-formed and not well-formed HTML that you’re talking about? Finally, I have no idea what “determine what should have been done and rewrite errors” means. Any clarification would be appreciated.