Re: region-based face-remapping

[Dmitry Gutov <dmitry@gutov.dev>]

On 06/01/2024 15:53, JD Smith wrote:
> In the world of CSS, you’d do this quite simply by (say) updating the 
> class of the div which wraps your text of interest, and having special 
> styling for, e.g., "alternate bold" and "alternate italic”, which is 
> only activated when the “alternate” class is applied:
> 
>     <div class=“normal”>
> 
>         This is <i>some italic text</i> and <b>some bold</b> text.  This
>         is unrelated text.
> 
>     </div>
> 
> 
>     In CSS
> 
>         .alternate b {...}
>         .alternate i {…}
> 
>     (In JS later:)
> 
>           myDiv.classList.add('alternate');

If we were to try to emulate CSS's nesting capability, perhaps we would 
add some new attributes which would alter the existing ones (using 
relative values). E.g. :background-relative and :foreground-relative, 
which could be used to lighten or darken the colors provided by the 
:background and :foreground attributes. The -relative ones could come 
from a separate face, but the question would be how to make sure to 
apply it - it would have to be the last rule in font-lock rules, to 
avoid forcing all other rules to use :override. Or, I don't know, if 
would need to be applied some other way at the end of font-lock. Main 
thing, it wouldn't work off an overlay.

Or a different spin, for better theme-ability: some declarative forms 
which allow to specify resulting attributes (colors, etc) for 
combinations of faces. I.e. if two faces are present together, the 
foreground and background should be this and that. The limitation is 
also like the above.

But to be able to set these faces through an overlay, or even a text 
property but without worrying about the application order vis-a-vis 
font-lock, it would probably need to be a new property. Not sure if we'd 
also need some new entities in its values, rather than regular faces.

Maybe the new property would specify one or several new values which 
would correspond to new conditions in the DISPLAY form of face 
definitions. But that would make defface forms incompatible with older 
emacsen, I guess, and it's not certain whether it would be easy enough 
to use for a third-party package author anyway.