[css-shapes-2] Ambiguity when creating an arc between angle & ry

[noamr]

See https://drafts.csswg.org/css-shapes-2/#typedef-shape-arc-command:
<arc-command> = arc <by-to> <coordinate-pair> of <length-percentage>{1,2} [ <arc-sweep> || <arc-size> || <angle> ]

There could be arguments that are ambiguous at time of parsing between the second length-percentage radius and the angle.
For example, unclear how to parse:
arc to 10px 10px of 5px calc(var(--some-number) * 2) cw

Is the calc value the angle or the y radius?

Even if it was possible for the parser to figure this out by following the calc parameters, this is ambiguous to the reader.

Options:

• Make it mandatory to have 2 radii, maybe allowing auto or so for the second one to represent symmetrical radii, like:

arc to 10px 10px of 5px auto calc(var(--some-number)*2) cw // angle
arc to 10px 10px of 5px calc(var(--some-number)*2) cw // radius

• add a keyword or a function(rotate?) before the angle, like:
  arc to 10px 10px of 5px rotate(calc(var(--some-number)*2)) cw // angle or arc to 10px 10px of 5px rotate calc(var(--some-number)*2) cw // angle
arc to 10px 10px of 5px calc(var(--some-number)*2) cw // radius

I think I prefer the rotate as keyword option. Maybe there are more options?