It's a feature that requires baking the hashes of the resource into the HTML. This JSDN talks about optimizing the resource separately for different browsers (e.g. adding polyfills where necessary), which means it doesn't just serve up a single resource. Subresource integrity support would require the ability to ask the JSDN for the hashes of all possible representations of the file, as well as the assurance that these hashes won't change (e.g. the polyfill won't get updated to a new version).