[css-align] Issues with <overflow-position>

Hi everyone,

While reading on the new properties, it seems that there are some
cases where <overflow-position> has no default or cannot be
overridden:

* <overflow-position> should have a default value for all types of
layout and all properties that is either true or safe (or else there
is a need for a third default value). Currently this is not the case
for at least the following cases:
   - justify-self with table cells and any layout that doesn't belong
to the one listed.
   - align-self in the same conditions and block-level boxes.
   - justify-items and align-items

* With the current syntax, it is possible to set the <item-position>
in such a way that you can't override the <overflow-position> in the
'baseline' and 'stretch' cases. This is because it's not allowed to
set the <overflow-position> for those 2 values and thus it defaults to
what is the default for the current layout.

Thanks,
Julien