I endorse this initiative.

One issue brought up in the specmeet (me/wjt/sjoerd): we want to be able to reference an entire alternative-group. We think alternative groups and identifiers should be disjoint, and taken from the same namespace, so that (e.g.): <a href="cid:catphoto"/> refers to:

* if there is a part with {"identifier": "catphoto"}, that;

* otherwise, if there are parts with {"alternative": "catphoto"}, that entire alternative-group.

(In reply to comment #2)
> One issue brought up in the specmeet (me/wjt/sjoerd): we want to be able to
> reference an entire alternative-group. We think alternative groups and
> identifiers should be disjoint, and taken from the same namespace, so that
> (e.g.): <a href="cid:catphoto"/> refers to:
> 
> * if there is a part with {"identifier": "catphoto"}, that;
> 
> * otherwise, if there are parts with {"alternative": "catphoto"}, that entire
> alternative-group.
> 

Oh heh, the definition of 'alternative' already says this. Merged.