There are a number of tutorials that suggest using the .mjs extension for working with ESM modules for front end development.

https://developers.google.com/web/fundamentals/primers/modules <https://developers.google.com/web/fundamentals/primers/modules>
https://hacks.mozilla.org/2018/03/es-modules-a-cartoon-deep-dive/ <https://hacks.mozilla.org/2018/03/es-modules-a-cartoon-deep-dive/>

There are many tutorials that suggest web developers use `python -m SimpleHTTPServer` as a way to spin up a quick development server.  If a .mjs file is not served with the correct mime type <https://html.spec.whatwg.org/multipage/infrastructure.html#javascript-mime-type> the browser will throw when trying to load the module, and the error is not the most obvious if developers are not aware of what mime types even are.

As such I would like propose that this is a bug fix, rather than a feature, as popular browser workflow is going to be broken without an obvious way to fix it for anyone using python right now.

> On Dec 5, 2018, at 2:23 PM, Christian Heimes <report@bugs.python.org> wrote:
> 
> 
> Christian Heimes <lists@cheimes.de> added the comment:
> 
> It's technically a new feature. New features are not added to existing releases unless there is a very compelling reason.
> 
> ----------
> nosy: +christian.heimes
> 
> _______________________________________
> Python tracker <report@bugs.python.org>
> <https://bugs.python.org/issue31715>
> _______________________________________