[css-transforms] UA stylesheet rule to give SVG elements a default transform-box value of view-box

[smfr]

https://www.w3.org/mid/op.x5otn9g7fwt8au@localhost.localdomain

CSS WG resolved to specify a UA stylesheet rule for SVG making the default transform-box for SVG elements be the view-box.

[smfr]

svg|* { transform-box: view-box; }

[smfr]

Even with a corrected UA rule, we still need prose about the used value to handle the case where the author set the transform-box to border-box.

Want to also have something that can be used at https://drafts.fxtf.org/fill-stroke/#fill-origin

[smfr]

svg|* > svg|* {transform-box: view-box}

[AmeliaBR]

We might as well follow the syntax in the user agent stylesheet from SVG 2, which sets default values of transform-origin. (Note that that user agent stylesheet uses a default namespace rule).

svg|*:not(svg), svg|*:not(foreignObject) > svg {transform-box: view-box}

The important net difference from the above is that SVG elements that are direct children of foreignObject elements are excluded.

In other words, SVG namespaced elements are set to transform-box: view-box except for the elements that have CSS layout boxes:

• root <svg> elements
• inline SVG elements that are direct children of non-SVG elements
• an <svg> element that is the direct child of a <foreignObject>

[AmeliaBR]

Per the resolutions referenced in #857 (comment), it looks like no new user-agent rules will be required. The different behavior between elements in SVG and CSS layout is going to be handled by the mapping of SVG boxes to CSS boxes.

[smfr]

Closing per above comment (now issue #999).