https://drafts.csswg.org/css-ui-3/#outline-color
Name:	outline-color
Value:	<color> | invert
Initial:	invert
Computed value:	The computed value for invert is invert; the computed value of currentColor is currentColor

Test pages:
http://jsfiddle.net/ericwilligers/ze1fbtnm/
WPT css/css-ui/inheritance.html

See also https://bugs.chromium.org/p/chromium/issues/detail?id=898803

We currently support the property but not the spec's initial value, which is unfortunate.