Digitally-signed SRI ?

Hi, I'm updating my web-security presentation for my net-sec class, and
think of covering SRI. There's a question I'm curios about. The draft uses
hash based authentication, but doesn't seem to offer an option for using
signatures. I can see a performance concern for the use of signatures
(validation, mostly), but in a common use case, signatures seem to be more
applicable (allowing a cached web-page to use periodically modified
resources from a not-fuly-trusted CDN, for example).  So I'm interested to
learn if this was a decision by the WG, and, if it was, what were the
considerations. A url to relevant email/thread would be helpful; I tried
searching the archive but in vain.

Many thanks! Amir
p.s. I'm sending this to the public mailing list but I'm not subscribed, so
please respond to my personal email, thanks.
--
Amir Herzberg

Comcast professor of Security Innovations, Computer Science and
Engineering, University of Connecticut
Homepage: https://sites.google.com/site/amirherzberg/home
`Applied Introduction to Cryptography' textbook and lectures:
 https://sites.google.com/site/amirherzberg/applied-crypto-textbook
<https://sites.google.com/site/amirherzberg/applied-crypto-textbook>