>> Are there further comments or questions about this patch?
>
> Yes.  I stopped responding because I didn't want to get in the way of 
> your finishing the merge of the locked-narrowing branch, but the 
> discussion about this one is far from over.  I will respond when I have 
> more time.
>

Okay.

To clarify the matter, I think the following is useful, to explain what 
"family = Sans Serif" means.

I attach a screenshot of the font preference box in Firefox.  In that 
screenshot you see the three most general font families: Serif, Sans Serif 
and Monospace.  The Serif and Sans Serif families are meant for variable 
pitch fonts.  Emacs uses these same "Sans Serif" and "Monospace" families.

Websites specify the font the browser should use in a specification, which 
should always include a font family, as follows:

font-family: 'Arial', 'Helvetica', sans-serif;

As you probably understand, this means "please use the Arial font for that 
element, or if it is not available the Helvetica font, or if neither are 
available fall back to the default sans-serif font".  Likewise:

font-family: sans-serif;

means "I don't care about the font, please use the default Sans Serif 
font".