You are thinking of the integrity="" attribute that's part of SRI: https://developer.mozilla.org/en-US/docs/Web/Security/Subres...

That attribute is meant to verify resource contents. The strategy of using it as a cache ID is still being discussed, but last I looked into it, there were some cache poisoning concerns.

You might find this thread useful: https://news.ycombinator.com/item?id=10311020