Media Source Extensions (MSE)[1] were added for YouTube HTML5 video, and is different from Encrypted Media Extensions (EME)[2], which is what allows DRMed videos.
MSE allows things like adaptive bitrate while streaming and programmable video buffering without a plugin.
MSE allows things like adaptive bitrate while streaming and programmable video buffering without a plugin.
[1] https://w3c.github.io/media-source/
[2] https://w3c.github.io/encrypted-media/