In 6.5.2 it says: The .minimum, .optimum, and .maximum components of the half-leading trait are set to 1/2 the difference of the computed value of the line-height property and the computed value of the sum of the text-altitude and text-depth properties. The .precedence and .conditionality components are copied from the line-height property.
If the line-height is smaller than the total text height we get a negative "half-leading" trait value. Is that a legal/sensible/plausible value or should it be forced to 0 in this case? For 2 of the 3 different line-stacking-strategies the half-leading trait becomes a space-before/after specifier on the line area and therefore becomes part of the space resolution algorithm. I assume negative values are OK in that case although I am not sure? Manuel