[css-transforms] `transform-box:stroke-box` with `vector-effect:non-scaling-stroke` creates a cyclical dependency

A cyclical dependency can be created if transform-box:stroke and vector-effect:non-scaling-stroke are both used on the same stroked SVG element. (And by implication, transform-box:border-box too, since the spec says "For SVG elements ... the used value for border-box is stroke-box.")

To calculate the stroke bounds of an element with vector-effect:non-scaling-stroke we need to resolve the transform to the element's outer-<svg>, but to resolve that transform when the element has transform-box:stroke-box requires the element's stroke bounds if the transform contains percentage values.

I don't think there's an ideal way to break this cyclical dependency, but I would suggest breaking it by requiring implementations to use fill-box in place of stroke-box (or border-box) if the element has vector-effect:non-scaling-stroke. (Probably regardless of whether it has a transform that contains percentage values, just to make implementation reasonable.)

Read the linked CSS Transforms Level 1 transform-box definition and the SVG2 Vector Effects section first, then review the issue's explanation of the dependency between stroke bounds and percentage transforms. Done means reaching and documenting an agreed specification rule that resolves this interaction without the cyclical dependency.