I've used the .mjs extension outside of Repl, and have used plenty of web apps that finally do too, but Repl doesn't highlight it at all, really discouraging it's usage.
XHTML files (extension: .xhtml) should be highlighted like that of .xml, not .html, as the highlighting for .html is severely broken. Also, it's a subset of XML, and has XML features, so .html highlighting makes no sense, but .html auto-complete would be nice, as .xml doesn't have any at all.
WebAssembly files (extension: .wasm) are binary files, why are they highlighted!? If there's a good reason, then I'll let it be, but the editor crashes upon editing them anyways, so it seems useless. And no one is about to edit a binary file by hand :/