RE: header-line and mode-line faces

[Drew Adams]

> > I don't think that is a good idea. The default value should 
> > just inherit from face `mode-line'.

> I don't see why.  Please explain more.

Uh, so you can customize just the mode-line face and have the header-line face
reflect the change. 
 
> > Users should be able to customize face `mode-line' and have face
> > `header-line' follow suite.
> 
> Why? they are different faces used for different portions of display.
> They should be different by default so people could easily tell one
> from the other.

They are different faces, and you can also customize them separately.

But header-line inherits from mode-line *today*. Why?

Inheritance provides convenience - these two faces are different, and have
different uses, but they have similarities. It's about convenience. Inheritance
by default never prevents anyone from customizing them separately.