[css-masking] "object bounding box" in SVG is the content-box, but UAs use the border box on CSS boxes

[smfr]

For transform-box and the geometry box argument to clip-path, fill-box maps to object bounding box for SVG elements. For CSS boxes, fill-box maps to the content box, which makes me think that "object bounding box" should map to the content box. However, browsers seem to map to the border box.

So we have ambiguity about whether "object bounding box" for CSS boxes in general should be the 'fill-box or content-box.