YouTube is planning to include a 'Subscribe Button' embedded within the Video itself. With the new subscribe button, users can easily subscribe to a channel without having to click through.
With this new update, YouTube is said to make the viewers life easier. In order to view the Subscribe button, you need to hover over the Engadget logo which is on the top left. Over here, you will see an option to subscribe to the specific channel.
As of now, users had to click to the video to YouTube's page in order to subscribe to that channel.