Looked at a few examples of spoiler tags and most of them were in the form of a Wordpress plugin or Javascript. I decided to write a CSS based one which required minimal effort to install and here it is:

Spoiler
It’s a bit wierd in the way it uses the a:focus dynamic pseudo-class but it works very well and looks very simple. One of the main problems however is that the CSS doesn’t match multiple paragraph tags but this will do for now.