This is mostly a placeholder, while I wait for the weekend to work on this in earnest.
TODO:
- X container & X tests
- X ring & tests
- X box-shadow & tests
- filter (but possibly as a function/mixin) & tests
- backdrop-filter (which will be difficult as neither Edge nor Firefox support backdrop-filter natively) & tests
- drop-shadow & tests
- animate-spin & tests
- animate-ping & tests
- animate-pulse & tests
- X screen-reader-only * tests