The new CSS Box Alignment specification introduces new values and syntax for the alignment properties, most of them not yet implemented by Flexbox. The case if the initial value is specially tricky, since it'd default, in some cases, to invalid values for Flexbox. We might protect the CSS Box Alignment implementation behind a compile flag (we can use the grid layout one, for now) in order to keep the old Flexbox's code path.
*** This bug has been marked as a duplicate of bug 161303 ***