Resolving `sources` for inlined (data URI) sourcemaps

[sapphi-red]

The current text describes that relative sources are resolved from the source map.

If the sources are not absolute URLs after prepending the sourceRoot, the sources are resolved relative to the source map (like resolving the script src attribute in an HTML document).
https://tc39.es/ecma426/#sec-sources

If the sourcemap is a data URI, IIUC relative paths cannot be resolved (e.g. new URL("./foo", "data:text/plain,foo") throws an error). So I think this description is not correct.

In this case, all tools I know resolves the sources from the URL of the file that contains the sourceMappingURL annotation.