A nice and fully CSS horizontal accordion
I used bootstrap and font-awesome in the example because I use them most of the time in my projects but you're free to use what you want.
The only file required is the smart-accordion.less. (css/bootstap/custom/smart-accordion.less)
I've done it quite fast so it's possible that some lines in the file are dedicated to the example.
The js file in the project is only the modernizr from HTML5 boilerplate.