Currently the spec of transform-origin contains the statement:

For SVG elements without associated CSS layout box the initial used value is 0 0 as if the user agent style sheet contained:

It would, on the face of it, be much more sensible to just define this as a UA style rule, rather than making UAs pretend.

(Given the wording "as if", does this mean WebKit's current implementation which uses a UA style rule is non-conforming?)