Style CSS Challenge!

Dear all, and especially any lurking CSS gurus,

One of the frequently expressed use cases for Style is to dim the rest
of the resource, rather than put a border or color over top of the
segment of interest.  Especially for images, where a semi-transparent
color could obscure the features that are being annotated.

So, the challenge ... is there a way using CSS to natively express
this sort of thing?  If not, do we need to add a second property to
give the class to apply to everything except the segment?  For example
oa:styleClass is the class to apply to the segment and
oa:inverseStyleClass is the class to apply to everything else.

Many thanks!

Rob