[css-shapes-1] Define <basic-shape> with CSS value definition syntax

[cdoublev]

I would like to get <basic-shape> defined using the CSS value definition syntax so that it can be be extracted by w3c/reffy, similarly as <color> is defined with its sub-productions.

<basic-shape> = <basic-shape-rect> | <circle()> | <ellipse()> | <polygon()> | <path()>

[cdoublev]

<basic-shape> is defined in CSS Shapes 1 with the following functions:

• circle(<shape-radius>? [at <position>]?)
• ellipse([<shape-radius>{2} ]? [at <position>]?)
• inset(<length-percentage>{1,4} [round <'border-radius'>]?)
• polygon(<'fill-rule'>? , [<length-percentage> <length-percentage>]#)
• rect([<length-percentage> | auto ]{4} [round <'border-radius'>]?)
• xywh(<length-percentage>{2} <length-percentage [0,∞]>{2} [round <'border-radius'>]?)
• path(<'fill-rule'>? , <string>)

CSS Shapes 2 extends this list with shape(<'fill-rule'>? from <coordinate-pair>, <shape-command>#).

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<basic-shape> is used in:

• CSS Masking: clip-path
• Motion: offset-path
• CSS Shapes 1: shape-outside
• CSS Shapes 2: shape-inside
• CSS Round Display 1: shape-inside
• SVG 2: shape-inside, shape-substract

For clip-path, CSS Masking links <basic-shape> to CSS Shapes 1 and does not restrict its value.

For offset-path, Motion links <basic-shape> to CSS Shapes 1 and does not restrict its value. path() and polygon() are problematic because they take <'fill-rule'>, which seems meaningless for offset-path.

For shape-outside in CSS Shapes 1 and shape-inside in CSS Shapes 2, the spec restricts <basic-shape> to inset(), circle(), ellipse(), polygon(). Why rect(), xywh(), path(), would be invalid?

For shape-inside in CSS Round Display 1, <basic-shape> is not linked and its value is not restricted.

Aside: CSS Round Dispplay 1 says that CSS Shapes 2 define the shape-inside property [...which] is extended with a new value named display, but it is already included in the value definition of shape-inside in CSS Shapes 2.

For shape-inside and shape-substract in SVG 2, <basic-shape> is not linked and its value is restricted to circle(), ellipse(), polygon(). The spec has Issue 76: do not re-specify shape-inside but reference CSS Shapes, and its value definition is different than in CSS Shapes 2. inset() is explicitly not allowed. Why? And does it mean that rect(), xywh(), path(), are not allowed? Why?

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I propose:

• to define in CSS Shapes 1 that basic shapes in CSS are represented by the <basic-shape> type, defined with <basic-shape> = <basic-shape-rect> | <circle()> | <ellipse()> | <polygon()> | <path()>
• to accept all <basic-shapes> for shape-outside in CSS Shapes 1 and for shape-inside in CSS Shapes 2
• to define that for all <basic shape>s, if they accept a <'fill-rule'>, it is ignored for offset-path in Motion

^{Edited suggested changes, after w3c/fxtf-drafts@b1fd570.}