Thanks Tav for pointing this out!
So instead of the "inkscape:transform-center-x" and "inkscape:transform-center-y" we have now, we would need to support "transform-origin". It's still expressed in terms of the geometric bounding box, but it's relative to the top left corner (instead of the center). So we need to change this anyway.
I still believe that it would be best to use absolute coordinates in Inkscape, both for ease of coding as well as for intuitive manipulation by the user. But we would then have a write a relative center to the file, complying with the CSS standard. That shouldn't be too difficult.
On top of that, the GUI for these manipulations needs some love. For example, in the selector tool the transformation center is only used for rotations and skewing, but not for scaling and stretching. For scaling and stretching, the corner or midpoint opposite of the bounding box opposite to the handle is used instead. This is not really consistent. I believe that for any transformation, it should be possible to use either the transformation center or the point opposite to the handle. There will be use cases for both, and we only have to provide a single toggle on the selector's toolbar. And while we're at it, we can add a toggle to reset the transformation center too (currently, the only way to reset it is to use the XML editor)
Diederik