The File and Directory Entries API simulates a local file system that web apps can navigate around. You can develop apps that can read, write, and create files and directories in a sandboxed, virtual file system.
The Web Animations API lets us construct animations and control their playback with JavaScript. This article will start you off in the right direction with fun demos and tutorials featuring Alice in Wonderland.
As web applications become more and more powerful, adding features such as audio and video manipulation, access to raw data using WebSockets, and so forth, it has become clear that there are times when it would be helpful for JavaScript code to be able to quickly and easily manipulate raw binary data. In the past, this had to be simulated by treating the raw data as a string and using the charCodeAt() method to read the bytes from the data buffer.
The original File System API was created to let browsers implement support for accessing a sandboxed virtual file system on the user's storage device. Work to standardize the specification was abandoned back in 2012, but by that point, Google Chrome included its own implementation of the API. Over time, a number of popular sites and Web applications came to use it, often without providing any means of falling back to standard APIs or even checking to be sure the API is available before using it. Mozilla instead opted to implement other APIs which can be used to solve many of the same problems, such as IndexedDB; see the blog post Why no FileSystem API in Firefox? for more insights.