I think 10% ends up being fairly neglible due to way the combat math works, but I can't think of another case of a negative modifier like that so I'm not entirely sure how it works. Generally speaking, combat promos get added to the base strength of the units, but all other modifiers are pooled together and cancel each other out. So when a C1 + shock HA attacks a C1 spear the 25% bonus just gets subtracted from the 100% bonus a spearman gets vs a mounted unit. If I'm not mistaken the HA ends up at 6.6 and the spear ends up at (4×1.1)×1.75 = 7.7.
Anyway, if the -10% gets added as a 10% bonus for the defender along with the other (generally large) pool of bonuses such as fortify, cultural defense, etc, it actually ends up making a relatively small difference. If it's calculated differently and actually subtracts 10% from the HA (like reverse combat promotion), it's a significantly worse penalty. That's not how I would guess it works, but if somebody does know for sure I would be curious to find out.