If an implementation does implement this interface, it is expected to
understand the specific syntax of the shorthand properties, and apply
their semantics; when the margin property is set, for example, the
marginTop, marginRight, marginBottom and marginLeft properties are
actually being set by the underlying implementation.
When dealing with CSS "shorthand" properties, the shorthand properties
should be decomposed into their component longhand properties as
appropriate, and when querying for their value, the form returned
should be the shortest form exactly equivalent to the declarations made
in the ruleset. However, if there is no shorthand declaration that
could be added to the ruleset without changing in any way the rules
already declared in the ruleset (i.e., by adding longhand rules that
were previously not declared in the ruleset), then the empty string
should be returned for the shorthand property.
For example, querying for the font property should not return
"normal normal normal 14pt/normal Arial, sans-serif", when
"14pt Arial, sans-serif" suffices. (The normals are initial values, and
are implied by use of the longhand property.)
If the values for all the longhand properties that compose a particular
string are the initial values, then a string consisting of all the
initial values should be returned (e.g. a border-width value of
"medium" should be returned as such, not as "").
For some shorthand properties that take missing values from other
sides, such as the margin, padding, and border-[width|style|color]
properties, the minimum number of sides possible should be used; i.e.,
"0px 10px" will be returned instead of "0px 10px 0px 10px".
If the value of a shorthand property can not be decomposed into its
component longhand properties, as is the case for the font property
with a value of "menu", querying for the values of the component
longhand properties should return the empty string.