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YouTube is now fully blocking ad blockers around the world
(9to5google.com)
This is a most excellent place for technology news and articles.
My guess is they can't afford the complication on the server side.
They could probably force the ad to be decoded by asking the client for proof. They couldn't prove that it was actually shown to the user though.
What I don't understand is why they don't block downloading the video during the period where an ad is supposed to be playing.