It seems that Google is working on a redesigned desktop video player that takes its influence from YouTube application. The player features transparent playback controls overlayed directly on the video rather than the old opaque black bar.
Like the current player, the controls disappear after a few seconds. Controls for settings like autoplay on playlists, annotations, and video speed and quality have also gotten a bit change.
In this new design there is also the omission of the “watch later” button, which allows users to save videos to a list for later viewing.
The change doesn’t seem to be available to everyone just yet. There is no information about when it will become the new standard.