Canvas2D ellipse() specs seem to be broken

[Kaiido]

[Originally reported at StackOverflow by user flupe.]

The ellipse-method-steps seem to be broken.
It looks like a monkey-patch over the old dom-context-2d-arc method definition, with some leftovers that kind of break the algorithm.

For instance, in the step 4 it says

If [...] startAngle-endAngle is equal to or greater than 2π, then the arc is the whole circumference of this ellipse, and the point at startAngle along this circle's circumference, measured in radians clockwise from the ellipse's semi-major axis, acts as both the start point and the end point.

Otherwise, the points at startAngle and endAngle along this circle's circumference, measured in radians clockwise from the ellipse's semi-major axis, are the start and end points respectively, and the arc is the path along the circumference of this ellipse from the start point to the end point

There is no previous mention of any "circle". So one could think that it's simply a typo and that it should be read "this ellipse" instead. But that doesn't seem quite correct either. These points aren't the ones at the angle startAngle or endAngle on the circumference of the ellipse, they are the points at the "eccentric angle" startAngle or endAngle. These angles do only match for actual circles, not for other ellipses.

Implementations do agree on what to do here, only the specs don't reflect what the implementations are actually doing.

I'm not quite sure how to make the actual algorithm in prose, so if someone from @whatwg/canvas (maybe @junov?) could give it a shot or at least provide some directions, that'd be great.

[annevk]

Reusing language from https://svgwg.org/svg2-draft/shapes.html#EllipseElement might make sense here.

[Kaiido]

<ellipse> is always a whole ellipse, i.e it doesn't have to deal with startAngle and endAngle.
The Arc command would have been a better candidate, but that one uses "endpoint parameterization", while canvas2D uses "center parametrization", see this note. I don't think we can reuse much from it to describe what's really done in canvas.