My question isn\'t so much about why a specific bit of jQuery I\'ve written isn\'t working as it is about no jQuery at all is working; not even working examples I\'ve copied
The way you load jQuery might be a problem. When you start your URL with // it is supposed to work when page is being browsed via HTTP or HTTPS protocols. However, if you will open it as a local file it won't work.
Given that your example works over HTTP I suggest you try to include jQuery as following: