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It depends on what you think YouTube's product is. Are they a service that streams videos and recommendations to consumers, or a service that streams eyeballs and tracking data to advertisers? The first one might get by with HTML5, but the second one definitely requires some JS.

Unless you want to minimize the video while you browse other videos.

Or upvote, downvote, and comment.

Or monetize the videos with ads.

Or a dozen other things.

Having to reload the page to like a video, add a comment or subscribe would definitely gut their engagement with creators.

Sure.

Now describe it from the perspective of a YouTuber.