The offset-anchor property says that percentages are relative to the width and height of "a box". The prose definition of the <position> value manually defines how to interpret a percentage, specifying that they're relative to "the content area". None of the examples have padding, border, or margin on the offset element, so it's impossible to tell what exactly is intended here, as all of the elements' boxes coincide.

Since 'offset-anchor' is clearly intending to match up with 'transform-origin' by default, I suspect we want to use the same box it does. I'm not sure whether we want to always use the initial value of transform-box or the computed value.

I suspect the best answer is the initial value (the "view box", aka border box for CSS). When you want to match the transform origin exactly, you can do that already with auto. I'm not sure what the use-cases would be to specify a different point than the transform origin that nevertheless requires using the same box as the transform origin. If we do realize we want to match this, we can always add a corresponding property with none | auto | <'transform-box'> as the value.