Optional and being able to require it with e.g. a linter on a per-project basis seems better idea for me, since I'd strongly prefer not using & and for the vast majority of cases (as we learned from SASS) it's not needed.
Q for Smarter people than me with more experience: is it better to require stricter upstream and loosen the downstream (so would require &, and MyCSS framework could insert them where needed) or looser upstream and tighten local code with linters?
"be conservative in what you do, be liberal in what you accept from others" applies?
The problem is that "MyCSS framework" would not be a CSS framework, it'd be a full build system if it's incompatible, so that way no one would use the looser form. So if it's looser upstream, you can always tighten it more downstream in a fairly seamless and optional way, while the opposite is not really true.
