Twine is essentially a program that organizes web pages that will allow you to write non-linear stories, share prototypes and even animate short poems. It is a simple interface that begins with a selection of archived projects.

 

Once you click +Story on the right, you are presented with a blank window. From here, +Passage will create a passage, which is basically a page of your story/prototype/etc.

The +Tag feature allows you to select targeted CSS and JavaScript so that it specifically targets that page.

Twine lets you conveniently edit these two for your story from a menu on the bottom left:

The JavaScript and CSS written in these windows will apply to all the pages in the project.