Most likely a user-agent check to ensure compatibility. I don't assume any ill intent. That being said, what does annoy me is that we've known for years that feature detection is preferable to user-agent detection, and I would kind of expect one of the biggest web development companies in the world to be following best practices. This isn't my local pizza shop, Google engineers should know better. Have they already f…
Disclaimer: I work at Google but not on YouTube, also speaking just personally. This isn’t practical at all in a latency sensitive environment. Feature detection works once JavaScript has loaded on the page, which means we either have to serve you a giant bundle of stuff you may not be able to use, or we have to degrade the experience of the latest browsers by detecting features and fetching more HTML/CSS/JavaScript…
Google doesn't really care that much about latency, they only care about it so that they can hide from you that they are loading all that extra javascript that spies on you.
And btw, watching videos is not really latency sensitive, it is more bandwidth sensitive... (I don't really care if I have to wait 1 second to start watching a 2hour video, but I will be mad if it is buffering every 30 seconds)