em and ex units for "scriptminsize"

Are 'em' and 'ex' units for 'scriptminsize' to be interpreted relative to
parent font size? It seems they'd have to be, because interpreting them
relative to the current element's font size creates a circular definition.
But the spec doesn't say.

Rob
-- 
"He was pierced for our transgressions, he was crushed for our iniquities;
the punishment that brought us peace was upon him, and by his wounds we are
healed. We all, like sheep, have gone astray, each of us has turned to his
own way; and the LORD has laid on him the iniquity of us all." [Isaiah
53:5-6]