Your proposal might work as well, but it is incomplete as well, as a non-set attribute would call to inherit the value from the parent styles (which is not done yet). And given that the same problem also exists for the other optional attributes, and there the current temporary work-around solution is to just set default value, for consistency it seems simpler to me to just do the same here as well. No idea which one works better with real world documents, do not have many testing samples to compare.