It appears to be legal to say

validate type xs:untyped {...}

but we don't say what it might mean.

I suggest that we explicitly allow this construct and define it to have the same meaning as XSLT's

<xsl:copy-of select="..." validation="strip"/>

that is, it converts a typed document to an untyped one by removing all type annotations. There are a number of use cases where this is handy, for example before doing a deep-equal comparison.