Welcome to the forums.
I'm not sure that I see the connection between the meta keywords tag, and validation. I can see the use of a number of different elements that are essential to validation, such as including a document type declaration, a meta encryption coding, using opening and closing html tags, and head and body sections.
But when it comes to including a meta keyword element or not, I don't think that even matters when it comes to validation. The lack of such a tag doesn't mean that a page won't validate.
Is there something about this particular element that makes you think that it is essential to the validation of a page, because I just don't see it.
I'm also not sure that a page that does validate will rank higher in search engines then one that does. I believe in using validation as a way to mitigate against risk that there is some error that may cause a page to not rank well.
Here are pagerank 9 and 10 pages that do not validate according to the W3C validation program:
Here are the two pagerank 10 sites that I found that did validate:
Now we do know that pagerank is only one of a multitude of factors that counts towards how well a site ranks for specific queries. But all of the sites above tend to do pretty well in searches for subjects that are within the scope of their fields.
I suspect that an overwhelming majority of sites on the web fail to validate, and at least more than half don't even include proper DTDs or character encryptions. That's true with a number of those sites I listed above.
Frankly, I wish that validation were a factor in the rankings of sites. I suspect that in a number of cases, the errors that aren't being caught because people didn't bother to validate do hurt rankings. But, a site isn't more or less relevant because it has or hasn't been validatated.