For SVG elements without associated CSS layout box, the used value for content-box, padding-box, border-box and margin-box is fill-box.

For elements with associated CSS layout box, the used value for fill-box, stroke-box and view-box is border-box.

The Paint module uses a bit more sophistication in its box mappings: content-box | padding-box -> fill-box and border-box -> stroke-box. (The mapping in the other direction relates to text bounds, which is maybe not appropriate here.) At the very least, these two modules should be consistent in how they map the SVG and CSS terms where possible.