The specification currently states:

In collapsed-borders mode: the used border-width values of the cell (half the winning border-width)

Is this the correct way to account for the various changes that have gone into CSS2.1 regarding which border sticks out into the margin?

Additionally there is a note that says:

The margins are not included in the table intrinsic offsets because handling of margins depends on the 'caption-side' property.

But doesn't this break handling of percentage padding?