>>>> \baselineskip=0pt means that the height and depth of a text line is 
>>>> equal to the maximal height and depth of the characters in that line. 
>>>> See the attached two screenshots that demonstrate the difference.
>>>
>>> That's what Emacs does as well, when line-spacing is nil, but Augusto 
>>> says they are different.
>>
>> Unless I'm missing something, that's not what Emacs does, no.  I attach 
>> two screenshots of the exact same text with the exact same font, one 
>> with Emacs (setq line-spacing nil) and the other with TeX 
>> (\baselineskip=0pt). The effect is very different.
>
> Then your description of what TeX does is probably incomplete or 
> inaccurate.
>

The TeXbook says: "Whenever a box is added to a vertical list [i.e., a 
line is added to a page under construction], TeX inserts "interline glue" 
intended to make the distance between the baseline of the new box and the 
baseline of the previous box exactly equal to the value of \baselineskip."

In more detail, the line-spacing algorithm also uses \lineskip, whose 
default value is 1pt.  If you set both \baselineskip and \lineskip to 0pt, 
there is no vertical space whatsoever between the lines, see the attached 
screenshot.

>
> on the Emacs side, I can point you to the code which implements 
> line-spacing, which clearly shows that if line-spacing is nil, the 
> default, we add NOTHING (a.k.a. "zero") to the height of the line, 
> leaving it at its computed value of the sum of the maximum ascent and 
> maximum descent of the glyphs in that line.
>

Of course I trust you.  Perhaps Emacs has a different understanding of 
what the ascent and descent of a glyph is?  Otherwise a line of dots would 
use less vertical space than a line of X's, which is what happens with TeX 
with \baselineskip=0pt and/or \lineskip=0pt.

Anyway, I'm not sure all this is really important for this discussion.