Bootstrap is the most popular HTML, CSS, and JS framework for developing responsive, mobile first projects on the web. Here is the link to the official site:

getbootstrap.com

Bootstrap 4 was released (1st alpha version) on 19 August 2015. One of the changes I like the most about it is that the source code of the CSS uses the SASS prepro rather than LESS (I've always preferred SASS!), but there appear to be a lot of new features that will be nice, although I haven't explored them all yet!

To use this feature, copy the source code section below and replace the xxx with your content. If you need to have more collapsable tabs, duplicate one of the sections (e.g. lines #3 to #11 in the source code) and change all references to a number (e.g. "One") to a unique number - there should be FIVE changes like this per section (i.e. "headingOne" twice and "collapseOne" thrice).