Consider dropping IE10/11 support?

[simonsmith]

I think we can safely drop IE10 now, we might be able to remove a couple of the awkward hacks in utils-flex and grid. The biggest wins however would be from IE11 and transpiling custom properties. We'd no longer need to use :root or the rather convoluted naming for custom properties.

This is a big win in my opinion.

[mlnmln]

@simonsmith I think it is safe to drop IE10 by now. IE11 hovers around 2,5% still, so I'd say we would need to respect that unfortunately.

https://caniuse.com/#search=custom%20prop

Anecdotical: We recently (2019) had a project that had 37% IE11 users (local university).

Maybe we could think of creating a "modern" build, similar of how libraries like Vue.js are doing it nowadays, but for CSS?
https://cli.vuejs.org/guide/browser-compatibility.html#modern-mode