The spec says:

In this specification the color() function is extended to allow predefined color spaces for HDR [...]

More precisely, it extends <colorspace-params> and <predefined-rgb> with new alternatives and redefines the other productions with no change.

Would it be possible to only define <colorspace-params> and <predefined-rgb>?

This would avoid conflicts from duplicate CSS type value definitions exposed by w3c/webref, for CSS tools consuming this data. Resolving them is not always straightforward, even between different levels of the same spec.

Another step forward could be a value definition syntax like <extended> += new alternative that w3c/reffy could interpret as "New values" for <extended>, akin to the "New values" field in property definition tables.

This would prevent extended definitions from diverging unexpectedly and eliminate the maintenance cost of adding new values ​​to a production extended by another production.

There are not many extended productions though, so it might not be worth bothering the maintainers of w3c/reffy for this.