spec question - extent

I'm confused by the definition of the extent property:

"The root container extent is determined either by a tts:extent specified on the body element, if present, or by the external authoring context, if not present. In the former case, if the width and height is expressed in terms of two <length> specifications, then these specifications must be expressed as non-percentage, definite lengths using pixel units."

I'm not sure we should even be considering shaping the external root container, but if we are, then since body elements are notionally re-parented inside regions, it seems illogical to me that the body would specify the extent of the root container in which regions are placed. It would make more sense to me in such a case if extent applied to apply to layout and region, the former setting the authoring context, and the latter setting the regions within that context.

Sean Hayes
Media Accessibility Strategist
Accessibility Business Unit
Microsoft

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