Trouble with ES6 Modules

The error message here is

Failed to load module script: The server responded with a non-JavaScript MIME type of "text/html". Strict MIME type checking is enforced for module scripts per HTML spec.

This is a new requirement for web development which thus far could mostly get away with incorrect MIME types without much trouble.

Two quick web servers for local development which I know to have reasonable MIME handling are:

1. Python's: python3 -m http.server (see 1).
2. Node's http-server: npm i -g http-server && http-server (see 2).

In your case, the error message is not being shown, indicated by

Hide all […] 1 item hidden by filters

Fix this by clicking ‘Hide all’ and choosing ‘Default’ (and you may wish to set this to ‘All levels’ while working). Or reset the devtools to its default state:

1. Press F1 in the devtools (or choose menu > Settings).
2. Scroll to the bottom and click ‘Restore defaults and reload’.