Hey, as discussed documenting issues I stumbled over while trying to integrate with JSS:

1. Styled-system functions return value data structure.

   Currently, v4 returns a nested array, which could be a flat object as it has been before in v3. Seems like @jxnblk and I both want it flattened again 😂

2. Dependency on React prop-types isn't ideal, because it could be done in a universal way. Also it was a bit confusing to find propTypes property on those functions, because it made them look like a React component. I am looking to have a first-class support for styled-system and keen to integrate the validation automatically.

Related issue on JSS repo cssinjs/jss#974