[webappsec] SRI : allow multiple integrity attributes or ni:// uris?

The thread on caching got me thinking about ServiceWorker and how it
might interact with SRI.

One of the goals of SW is to transparently provide alternate
resolutions to a fetch, perhaps based on online/offline connectivity
or connection characteristics.  e.g. load HD images over Wifi, low-res
ones over metered 3G, and show a styled placeholder when offline.

It seems like it might be useful then to allow supplying multiple
integrity attributes to accommodate this behavior. e.g. "this video
should have hash ni://abc (actual content) or ni://xyz (you're offline
placeholder content).

Thoughts?

-Brad