On Tue, 22 Aug 2006, Patrick H. Lauke wrote:

> Along the same lines, what about sub and sup? To me, they seem dangerously 
> close to presentational markup, unless someone can enlighten me as to what 
> the semantic meaning of these two elements is...

The nature of subscripting and superscripting varies from purely 
presentational to strongly structural. It's a mistake to lump all kinds of 
usage under a simple model of just sub and sup.

> The examples given at http://www.w3.org/TR/xhtml2/mod-text.html#sec_9.12 . 
> aren't making it any clearer:
>
> H<sub>2</sub>O
> E = mc<sup>2</sup>

Those are traditional examples for HTML, but this does not make them any 
better. The former is "semi-structural". The meaning normally remains the 
same if <sub>2</sub> is replaced by mere 2, but subscripting is still part 
of the notation system and not just esthetic styling, and in some contexts 
it makes a semantic difference.

The latter uses strongly structural markup. To make the point even 
clearer, compare 2<sup>2</sup> by 22. The example is in error, since it 
purports to be physics formula but fails to use italics for the quantity 
symbols; it should be <i>E</i> = <i>m</i><i>c</i><sup>2</sup>, unless you 
define some semantic markup for quantity symbols. It's not a matter of 
styling; in physics, upright (Roman) "m" is a unit symbol, italicized "m" 
is a quantity symbol.

> Surely this should be marked up more rightly with something like MathML?

MathML is hopelessly complicated and thoroughly confuses structure and 
semantics with rendering. Too bad HTML 3 was never approved, or even made 
a complete draft; if implemented, it would have given something _useful_ 
in the area of mathematical markup.

> <span xml:lang="fr">M<sup>lle</sup> Dupont</span>

That's of course purely presentational (which doesn't mean irrelevant),
just as 1<sup>st</sup> vs. 1st is for English. So should it be eliminated? 
Do we really want to force people into writing foolish "structural" markup 
like M<span class="sup">lle</span>?

-- 
Jukka "Yucca" Korpela, http://www.cs.tut.fi/~jkorpela/