> 1) <div> and <span> aren't sufficient, because I can't nest
>     a <div> inside a span.  In practice, I'd want to be able
>     to nest two or more divs ionside a single span, in order
>     to be able to typeset in columns.

That's abusing HTML as a page description language.

> 
> 2) AFAIK, there /are/ no "methods of extending the language
>     in a fairly orderly and standard way, in any direction
>     that the author or a community wants", and this I regard

That's why XML is called *X*.  You can define additional elements, as
long as you maintain the well-formedness (which exists to support this,
not to avoid confusing people with implicit tags).  You can use namespaces
to avoid conflicts.

[X]HTML is very much a lowest common denominator.  The original concept
was similar to the current Wiki concept, i.e. that it provided something
that any intelligent person could create and edit.  There are other
XML applications, like Docbook, that are designed for more sophisticated
document creation.