I fail to see how it is in anyone's best interest to implement support for list-style and column sizes on an individual <references> list.

These are traditionally implemented in a {{reflist}} template on Wikipedia. That was by all accounts a compromise due to the limited capability they have inside a template. What people want is for stuff to just work. They don't actually want to specify it manually each time.

Adding this to the extension will improve nothing other than move it from a template hack to something we now have to support and maintain forwards.

There has yet to be given a valid reason for why anyone would explicitly, semantically, by intent, want one specific article to have its references in a certain number of columns or certain list item number style.

The number of columns should be determined by width, not hardcoded count. And while the latest patch uses width, it is still imho fundamentally flawed. Users don't want to hardcode this everywhere (authors will not know this even exists, leading to generally no single user actually using this feature other than power users, whom then have to write bots that fix it everywhere; or more likely, they'll just use another template that specified the width and list-style they agree on for that wiki).

If this were any other feature, it'd be obvious this is something for the software to provide as a convenience. Not a burden for the users. Based on design considerations, can make a reasonable choice for what the column width should be. And it can be improved as we collect more data and positively influence all readers.

While there may not be a perfect value, I know for sure the factors that make it less perfect don't vary by article. They vary by device, browser, etc. It will traditionally and up with authors using their personal preference for what looks right on their device.

The only thing that is probably in need of variation is list-style (based on content language), which, again, seems like something we can be much smarter about. If there isn't a way to get that already from the Language infrastructure, we can provide a config variable or mediawiki-message specifying the list-style for that wiki or language. One exception could be multi-lingual wikis, so maybe provide list-style as attribute indeed.

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Gabriels proposal for css classes is imho better than providing a hardcoded column-width, but per the above, we don't need neither.