Reported by Øyvind Stenhaug
Value assignment on shorthand properties is not well-defined. For example, some combinations of longhand property values cannot be represented by a value on the corresponding shorthand property:
What is the computed value of 'border' given the following computed values of longhand properties?
Conversation begins: http://lists.w3.org/Archives/Public/www-style/2011Oct/0473.html
Bug description: http://lists.w3.org/Archives/Public/www-style/2011Oct/0476.html
Gecko apparently doesn't handle getPropertyValue on computed style for shorthand properties in an ideal manner (cf https://bugzilla.mozilla.org/show_bug.cgi?id=137688 , http://stackoverflow.com/questions/4230022/get-border-value-with-getcomputedstyle-getpropertyvalue-mozilla-ff ) which may be as a result of this issue.
Related: some implementers think that shorthands should "just work" so that there's no noticeable affect if a property changes from being a longhand to a shorthand property, unless the value specified is one that can't be specified in the original (now shorthand) form. (eg: http://lists.w3.org/Archives/Public/www-style/2010Feb/0104.html )
That does doesn't solve the hard case described, though.
Bug 15387 is another example of this bug.