If you really have your heart set on a button I would try the .hta approach. When the user double clicks on it, it opens just like a text file opens the default editor.
Only it's considered an "application" on the system. You can read more here:
Introduction to HTML Applications (HTAs)
Your javascript spoiler button should work in it.
edit: the wysiwyg editor does get stuff on a page quickly. But looking at it afterwards is the tough part. They tend to stick a zillion " "s in the page to space things. But look around. You may find a free HTA "toolkit" on the web someplace.
Also there are programming source code repositories for VBScript and JavaScript. You might find examples that can be modified or fleshed out to get what you want. Also those programmers will answer questions on the forums to help you.