This is a great feature.

But I have a few questions regarding user preferences,
Is it possible for a forum user/account to set their own preference so that every spoiler (hidden content) is either always hidden by default or always displayed?

And maybe not every user will notice on time, so is there a way to add hover feedback to the "show" button so that when someone who isn't aware hovers over it, a short text in the form of a tooltip will appear. I some something like that in the text editor.