[css-logical-props] handling values that are valid in one orientation but invalid in another

This might be a hypothetical question for now.  How do we handle logical 
properties whose corresponding physical properties don't take the same 
set of values?  Will we have a general rule to handle cases like this?

Gecko currently supports prefixed versions of {min,fit,max}-content 
values on the width property but not on the height property.  If I want 
to continue that arrangement, and I want to implement the new 
{block,inline}-size logical properties, how should I handle for example:

   div { writing-mode: horizontal-tb; block-size: -moz-min-content; }

Presumably this parses OK and you can inspect the block-size property's 
value on the declaration.  But at computation time, what value should 
the height property get?

One answer would be to make it the initial value; this is what happens 
with properties that have variables that don't resolve to a valid value.