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.
The <audio> and <video> elements provide support for playing audio and video media without requiring plug-ins. Video codecs and audio codecs are used to handle video and audio, and different codecs offer different levels of compression and quality. A container format is used to store and transmit the coded video and audio (both together, the case of a video with a soundtrack). Many combinations of codecs and container formats exist, although only a handful of these are relevant on the Web.