[kaelig]

Colors can be expressed in many ways. For example in Android it's common to see hex codes as #AARRGGBB, but in CSS the alpha is at the end (#RRGGBBAA). With wider gamuts (lch, dcip3...), there are separate channels and alpha is expressed separately. We also need to provide ways for folks to codify dark mode / light mode / high contrast values.

Another example is what we call "aliases" or "references": a token can reference another one. Their resolution process needs to be specified (as in: when exactly does an alias get resolved in the lifecycle, and how tools must process them, whether it's okay to rasterize them in the CSS/XML/JS/.. output, etc).

Note that we intend to provide a JSON schema, and the community has already published a few TypeScript type definitions, linting tools, and build tools based on the spec.

> some are pixels or rem or whatever

The 'or whatever' part is what we're trying to tame. For example an Android app may need to consume 'dp' values, web 'rem', iOS 'pt'... There are tons more examples where platforms differ in how they would express dimensions, typography, color... The spec provides a way to encode the source tokens, and then translation tools handle the conversion to platform-specific values.