The syntax description on page 11-154 of the PFD spec would indicate that either var or varDom must be specified when using the action. If using scopeDom is specified, varDom must also be specified.
Ryan - are you sure you're looking for the right kind of error? In the current RI, this constraint happens to be enforced by a TEI, not a TLV. I believe the RI is compliant in this regard. For <x:parse scopeDom="foo"/>, I get the following translation-time error: org.apache.jasper.compiler.CompileException: /xml/foo.jsp(1,0) Attributes are i nvalid according to TagInfo