This occurs because `youtube.com` doesn't have access to cookies on about.google. Typically the watch later button sends a request to `youtube.com/edit_playlist?key=USERS_UNIQUE_WATCH_LATER_ID`. This can't work without access to YouTube's cookies (the precise mechanism is unclear). We block youtube's cookie access as a third party because of its inclusion in the Disconnect Content List. This can be mitigated if the YouTube iframe requests Storage Access. However, I don't think we should intervene and shim for an entity blocked by =
Bug 1754431 Comment 1 Edit History
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This occurs because `youtube.com` doesn't have access to cookies on about.google. Typically the watch later button sends a request to `youtube.com/edit_playlist?key=USERS_UNIQUE_WATCH_LATER_ID`. This can't work without access to YouTube's cookies (the precise mechanism is unclear). We block youtube's cookie access as a third party because of its inclusion in the Disconnect Content List. This can be mitigated if the YouTube iframe requests Storage Access. However, I don't think we should intervene and shim for an entity with cookies blocked by the Disconnect list.