I copied your code to hide a fieldset until checkbox checked by wrapping the fieldset in a div and it worked but I don't understand why it works.
I understand the CSS to hide it. I understand the html for the div, the input, and the onclick.
What I don't understand is:
1. (box) - is box just a variable name or is it some inherent code for checkbox?
2. box.checked?"block":"none" - what are these called? nodes? attributes? I want to know what tutorial to google to learn how to understand and do this. It works like an if else statement but severely condensed. I'm amazed but it's not hard to amaze beginners.