Well, it really seemed to me that changing the principles of an organization might warrant some institutional thought (eg. in the form of a ticket), but hey if not I'll just add it.

I just want to be clear: you're rejecting the principle that MediaWiki is for everyone because ... of a very specific interpretation (evidently that saying "MediaWiki is for everyone" = "MediaWiki must support Internet Explorer 0.1").

What necessary function is broken?

I should note that there are some within the MediaWiki organization who believe this should instead state:

I will re-open if I can get https://phabricator.wikimedia.org/T232953 through.

According to https://www.mediawiki.org/wiki/Compatibility#Browser_support_matrix , IE versions 6-10 are under "Basic (Grade C)" which already means that "these browsers do not get JavaScript features."

With all due respect, the difference between this ticket and T178356 is the difference between saying "I'll fix things for a (small) category of users" and saying "screw those users!"

P.S. Another way people may find this same functionality useful is in CSS. Both the popular LESS and SCSS libraries involve doing the exact same thing with CSS code (passing it through a "translator" to output browser-readable code but let people write non-browser-readable code first).

What exactly is the "this" in "accomplish this"? :)

Honestly I wasn't sure myself. As I said, I started with a Super User question for that very reason, but then when it didn't get any responses I brought it here.

How performant does it need to be? This operation only has to be run after a Javascript file is saved, not every time it is served.