Useful applications of polyglot files that would unironically be good to see people adopt:

- JS-based build scripts being replaced with HTML/plaintext polyglot files that serve as both the README and the build script and that run in the universal runtime (the browser) rather than requiring e.g. NodeJS; alternatively

- JS/JS polyglot files that you can run with NodeJS if you so choose but that have first-class support for the browser instead

- JSON/HTML+JS polyglot files that parse as valid JSON but that you save with a .htm(l) extension and provide an advanced browser-based viewer for the data encoded in the JSON (especially typed data, e.g. JSON-LD)