[css-fonts] What does font-size-adjust for '0' look like?

Hi Everyone.
I have a simple question on font-size-adjust. [1]
According to the spec. font-size-adjust can have ‘none’ or <number> for its value. 
BTW what is expected to look like where font-size-adjust is ‘0’? I checked Gecko(which is the only vendor supporting for the property now) treated it as ‘none’.
Does it make sense? As my reading, font-size-adjust represents an aspect value (x-height / font-size) of a font.
So ‘0’ might mean x-height is 0 or equivalent very small value here. Thus It should be same effect with 'font-size = 0px’
I’d like to clarify this before landing a relevant patch for blink [2]

Best regards.

[1] http://dev.w3.org/csswg/css-fonts/#propdef-font-size-adjust <http://dev.w3.org/csswg/css-fonts/#propdef-font-size-adjust>
[2] https://codereview.chromium.org/983073002/#msg23 <https://codereview.chromium.org/983073002/#msg23>
ChangSeok