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There is an RFC 2119 erratum that modifies the keyword phrase intended to be used with conforming documents. The RFC inadvertently left the phrase "NOT RECOMMENDED" out of the message that conforming documents should display. The erratum linked in the URL field attempts to correct this omission. However, DOM4 has not made this correction.
https://github.com/whatwg/dom/commit/96fe9763208a92cdb03958b1da048807e878173c