Technically, they "work" in both scripts and modules, they just work differently in each case. This is valid Javascript that returns true if running in script mode or false if running in module mode:

    const isScript = (x = true) => x <!--x
    console.log(isScript()) // true or false depending on whether the code is ESM

What's happening is that `<!--` is a comment token in script mode, but it's parsed as `< ! --` in module mode.