I noticed that negative values are re-encoded with double-negative signs, which breaks the bidirectional conversions for full CSS. For example, trying to parse and re-serialize .card { margin: -1rem }:

require 'minicss'
root = MiniCSS.parse(".card { margin: -1rem }")
puts MiniCSS.serialize(root)

This outputs .card { margin: --1rem }

I think it's because the Number's value is -1.0 and the sign is -, so when the serializer processes it, it outputs both the - string and the - from the negative value.