There is an implementation difference on what should happen for omitted subproperty value in shorthand.

Currently, Gecko and Edge reset missing subproperties in shorthand to their initial value as a declared value, while Blink and WebKit sets them to the initial keyword.

The spec says:

When values are omitted from a shorthand form, unless otherwise defined, each “missing” sub-property is assigned its initial value.

so it seems to me Gecko and Edge match what the spec says. But I can see that the text here can be easily misunderstood given the existence of initial keyword.

Can we add a note to the spec to emphasize that it is not resetting missing subproperties to the initial keyword? This may be worth adding a new CSSOM test into wpt.

I found this when I'm fixing Servo's code, which somehow largely followed the WebKit's way, and thus fails lots of tests in Gecko. It seems to me setting initial value to subproperties makes serialization code more complicated because individual shorthands would need to take CSS-wide keywords into consideration as well (and consequently lots of bugs in Servo serialization code in edge cases), so I don't want to adopt WebKit's approach here.

There is no known web-compat issue so far, but this is a detectable difference, so I also hope Blink and WebKit can fix this before any real web-compat issue appears.