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Most YouTube Videos have keywords. If they are allowed, all is fine. Else, blocks with a page displaying the keywords.

Almost all youtube videos have keywords. If the keywords are in the allowed-list (a regular expression), all is fine. If they are not, the page is blocked and its keywords are shown.

This is a strict filter that only allows certain keywords. For one that is more lenient, have a look at The Video Blocker extension.