Proposal for @selector attribute on rendition

This proposal arises out of FR 520, which presents a number of ideas for enhancements to the element. This proposal is the simplest and most straightforward, consisting of a single new attribute, which I believe will be the least controversial and easiest to implement.

Rationale
The combination of and @rendition currently allows us to link specific elements with blocks of style code (almost always CSS) in the manner of the HTML @class attribute, and @style allows the direct application of CSS to a specific element. What we cannot currently do is specify a default rendition for an element or a set of elements, except through the use of /@render; this is crude in allowing only the linking of style blocks with unqualified elements, and it also brings in the requirement that every element used be provided with a matching  element.

I would argue that a better approach would be a new attribute called @selector available on the element, through the att.styleDef class. Its definition would be:

A selector or series of selectors specifying the elements to which the contained style description applies, expressed in the language specified in the @scheme attribute.

This would allow simple definitions like the following:

 display: block;

making this a straighforward way in which to express a basic mapping between TEI elements and their default styles. It would of course allow much more sophisticated mapping, including for instance the use of @rend:

 font-style: italic;

and would make it easy to express different default rules for the same element in different contexts:

 margin-top: 0.5em;

 margin-top: 0;