There are two issues at play. The first is that the importer may not have defined what a public_url means. The second issue is that even though the semantics of a public_url for filesystems is fairly well defined, it's possible for there to be no way to construct the public url for the filesystem paths in use. E.g. If the sourcemap is being generated into a different disk (on windows) than the stylesheet. In this case, the per-uri warning makes sense to me. I've separated this into two warnings. There's the default warning about implementing the method, but there's also a warning in the filesystem importer.