Steps to reproduce

1. Open a tab on https://ffx-external-import-map.glitch.me/
2. Open the console

Result

As I was testing out importmap, I tried declaring as we do for external script:

    <script type="importmap" src="map.json"></script>

In such case, I do get an error on the console as the importmap isn't loaded, and thus the import fails

Uncaught TypeError: The specifier “square” was a bare specifier, but was not remapped to anything. Relative module specifiers must start with “./”, “../” or “/”. module.js:1:27

But I don't get any error about the importmap being invalid (unlike in Bug 1808357).
It would be nice to have an error (or warning) message to indicate that the importmap can't be specified in an external file and should be inlined

Per spec, the onerror handler of the script tag will be called if it's external script with type is "importmap".

See
https://html.spec.whatwg.org/multipage/scripting.html#prepare-the-script-element
Step 30.1

(In reply to Yoshi Cheng-Hao Huang [:allstars.chh][:allstarschh][:yoshi] from comment #1)

Per spec, the onerror handler of the script tag will be called if it's external script with type is "importmap".

See
https://html.spec.whatwg.org/multipage/scripting.html#prepare-the-script-element
Step 30.1

That's good to know but do require people to know about the onerror handler too. Having a warning message would be a nice way to reduce frustration for users so they don't have to search around what's going on

I will try to dump some error message in console.

https://hg.mozilla.org/mozilla-central/rev/302bffceb24a