As far as I understand, CSS Modules Values syntax is designed to not break CSS.

Its main syntax (to define a value) is @value my-color: #ff0000; or @value my-color: #ff0000;. The idea is that variables can be preprocessed into these value definitions, as follows:

:root {
  --my-red: #ff0000;
}
@value my-color: var(--my-red);

In this example postcss-custom-properties fails to recognize and parse the var(--my-red).