The Computed value for mask-position seems incorrect or at least inconsistent with regards to the definition of background-position.

Currently the definition states:

Consisting of: two keywords representing the origin and two offsets from that origin, each given as an absolute length (if given a <length>), otherwise as a percentage.

This reads to me as e.g. left top 10% 5px or left bottom 5px 10px. Or possibly as left 10% top 5px. Observed behavior in the major browser engines seems to be an implied left-top corner and only explicit length-percentages, i.e. 10% 5px. This matches the way I read the corresponding description of background-position (emphasis mine):

list, each item a pair of offsets (horizontal and vertical) from the top left origin each given as a computed <length-percentage> value

So I think the definition for mask-position should be aligned with the one for background-position.

Note that WebKit and Blink show some inconsistencies around this, but those are the same inconsistencies that can be seen on background-position-computed.html