In gradients or linear(), you don't need to specify any of the color stop positions. The first defaults to 0%, the last to 100%, and anything else gets equally spread out between their surrounding specified positions.

Currently, the *-interpolate() functions don't allow this, because it's not clear how we want to write the grammar for it. Each "stop" is written as <input-position>{1, 2} : <output-value>, so merely omitting the position would make it look like:

width: calc-interpolate(75%,
  : 200px,
  : 500px,
  : 1000px );

...which looks pretty weird, imo. (This would be equivalent to 0: 200px, .5: 500px, 1: 1000px.)

For everything but interpolate() itself, it's just fine grammatically to omit the position and colon entirely, like calc-interpolate(75%, 200px, 500px, 1000px). But interpolate()'s outputs are <whole-value>, which can theoretically contain colons. However, we already have to use the {}-wrapping for outputs that include commas; we could rely on that here too.