Media Source Extensions (MSE) is a W3C specification that allows JavaScript to send byte streams to media codecs within web browsers that support HTML video and audio.
[5] Among other possible uses, this allows the implementation of client-side prefetching and buffering code for streaming media entirely in JavaScript.
It is compatible with, but should not be confused with, the Encrypted Media Extensions (EME) specification, and neither requires the use of the other, although many EME implementations are only capable of decrypting media data provided via MSE.
[6] Netflix announced experimental support in June 2014 for the use of MSE playback on the Safari browser on the OS X Yosemite beta release.
[8] Firefox 37 already had a subset of MSE API available for use with only YouTube in Firefox 37 on Windows Vista or later only,[9] while Mac OS X version had in enabled starting version 38.