link appearance and soft face properties

[Drew Adams]

We've been discussing whether or not links should be underlined, whether or
not link underlining should be applied to the link face or its mouse-face,
and whether or not link appearance should be the same in all buffers.

Just a thought, for consideration after the release:

What about having a :link face attribute (property) that is soft, not hard?
This would be analogous to using soft markup like Emphasis, as opposed to
hard markup like Bold: Emphasis can be different for different
users/applications/environments, whereas Bold always means bold.

That is, in order to separate purpose/intention/use from physical
formatting, why not have soft face attributes like :link that a user can
define/override (e.g. in terms of faces or face attributes)? IOW, why not
separate identifying something as a link from defining its appearance?

That way, a single (possibly user) definition of the :link attribute would
automatically affect any faces that have that property. And it would also be
easy (including for users) to selectively apply the :link property to face
or mouse-face or both.

Discussion here of the appearance of links (underline vs this or that) would
then be reduced to discussion of 1) the default appearance of the :link face
property and 2) whether to apply :link, by default, to face or mouse-face or
both (and, perhaps, in what buffers). Less heat, more flexibility.

This would also carry over to other soft face properties, having an impact
on the appearance of things like the mode-line and buttons. Instead of
hard-coding the appearance of everything, we would define a default
appearance using high-level property classes like :link.

Note: I'm not that familiar with face definitions, face
attributes/properties, and face inheritance, so don't get excited if this
makes little sense. In particular, perhaps this functionality is already
available via face inheritance or in some other way. In that case, what
about using this functionality to implement a generic "link" face or face
property that would then be inherited by the appropriate faces that are
today used for links?

IOW, such properties appear today to be hard-wired - why not look for some
way to soft-define them? A link is, after all, a user-interface object that
exists independently of its particular formatted appearance. In Web
browsers, users can easily decide whether links should be underlined, green,
etc. or not. Why not give that same flexibility to Emacs?