That would mean loading the content even if it isn't viewed. Although very handy script to have, handy for Q&A sections or when you want to give the option to toggle an area on and off.
I have actually just plonked a simple one into a site i'm working on in the news section, as the news is pulled from a database the ID of the news is given to the content area of each news post and a toggle link shown next to the news title.
This means if you have a list of lets say 100 artciles you could essentially have the page load with all content elements collapsed and have them open onclick. However the problem i see here is you are still loading that content, it's simply just hidden until the toggle is clicked. So i doubt it's something i'd consider using on a page that generates a lot of content, as such i will limit the SQL query to say 5 latest posts, and have the toggle as an added extra for those working on smaller screens and what to toggle content rather then having to scroll. This works good for me since i work inside a fixed width design, which i know some people are against i personally think it depends on the look you go for and the content being displayed.
If you want dynamic content then ideally you need to look beyond HTML and CSS into the more advanced languages. Personally i'm slowly learning PHP and now also some MySQL which is really making my head hurt.
In regards to the opening post, simple HTML includes will be sufficient and has very little requirement to function on both the server and client side of things...