[css3-fonts] computed style for font-feature-settings?

Earlier I referred to the potential need for an OM API to enumerate the
features supported by a platform or a font. There is a somewhat related
question, which is: what features should be enumerated when obtaining the
computed value for font-feature-settings? Should every feature of the font
be returned with the current value? Should, in addition, features supported
by the platform but not the font be included? Should the return value also
take into account side-effects on features from other properties, such as
font-kerning: none or font-variant-ligatures: none [which can have such
effects as causing 'kern' 0, 'liga' 0, etc.)?