Edit: just reread the GP and saw that they said the content should be able to come from a cache of a different domain. You're right, hash collision there would be bad (as others had mentioned you'd want something with a low chance if collision construction).
How does this introduce a new XSS issue? It's no different from the current system. And in this case the hash isn't being used to verify the content, it's there for cache invalidation.
I guess someone could poison your local cache somehow, and maybe that could be a problem with shared cdns. But there are other mechanisms to make sure you're delivered the right content in the first instance.
I think the main issues are actually browser support and having to deal with it in your own framework / code. That's the point people start saying - you know what, the existing system works well enough, I'll jut crank up the expiry on my existing headers and stick a cache buster in the URL when it needs to change.
How does this introduce a new XSS issue? It's no different from the current system. And in this case the hash isn't being used to verify the content, it's there for cache invalidation.
I guess someone could poison your local cache somehow, and maybe that could be a problem with shared cdns. But there are other mechanisms to make sure you're delivered the right content in the first instance.
I think the main issues are actually browser support and having to deal with it in your own framework / code. That's the point people start saying - you know what, the existing system works well enough, I'll jut crank up the expiry on my existing headers and stick a cache buster in the URL when it needs to change.