[css-align] details for justify-self on blocks should refer to margin properties, not offset properties

[dbaron]

The details for justify-self on blocks say:

In terms of CSS2.1 block-level formatting [CSS2], the rules for “over-constrained” computations in section 10.3.3 are ignored in favor of alignment as specified here and the used value of the offset properties are therefore not adjusted to correct for the over-constraint.

Here (unlike in the following section on absolutely positioned elements), this should refer to the "margin properties" rather than the "offset properties".