Is the keyword meta tag required?
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Is the `keywords` meta tag required in an html document head?

It seems to me it is not, from the W3 description of the document structure: http://www.w3.org/TR/html401/struct/global.html. But I can't find anywhere where they say it directly.

The reason is that I have heard people argumenting that it should always be present. Even empty. My question is not regarding the usefulness of the tag, but just if it is required or should always be present of some reason or not.