It's against YouTube's terms and conditions:
Section 5L
You agree not to access Content or any reason other than your personal, non-commercial use solely as intended through and permitted by the normal functionality of the Service, and solely for Streaming. "Streaming" means a contemporaneous digital transmission of the material by YouTube via the Internet to a user operated Internet enabled device in such a manner that the data is intended for real-time viewing and not intended to be downloaded (either permanently or temporarily), copied, stored, or redistributed by the user.
Section 9
"Content may not be downloaded, copied, reproduced, distributed, transmitted, broadcast, displayed, sold, licensed, or otherwise exploited for any other purpose whatsoever without the prior written consent of YouTube or, where applicable, YouTube's licensors. YouTube and its licensors reserve all rights not expressly granted in and to their Content."
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