[filter-effects] initial color for drop-shadow() filter function

Hi,

Currently drop-shadow() is defined as box-shadow but without inset[1]. It came up a question what this means for the color value of the drop-shadow if it is not defined:

filter: drop-shadow(3px 3px);

Apparently WebKit and Blink use transparent black as shadow color. Should the color value assumed to be transparent black if missing? Should it be another color like black? Or should it be the current value of the 'color' property as defined for box-shadow and text-shadow? I would prefer the latter.

Greetings,
Dirk

[1] http://www.w3.org/TR/filter-effects/#FilterProperty