See: #12315

I also found it a bit surprising that there is no way to have local typed custom properties. (except result)

@property --x {
  syntax: "<color>";
  inherits: true;
  initial-value: pink;
}

@function --foo() returns <color> {
  --x: rgb(255 0 0);
  
  result: if(
    style(--x: red): green;
    else: black;
  );
}

* {
  background-color: --foo(); /* black */
}

What was the original motivation to make everything untyped inside custom functions?

Because values are resolved in imaginary child elements nothing leaks out when a function is evaluated.
I can imagine that when the types are preserved it is possible to break custom functions by registering seemingly unrelated properties.

So maybe it should be possible to do local registrations?