Yet another validity question

[Richard Tobin]

> I didn't see anything in the spec that explicitly says "you must (or don't
> have to) declare all elements referenced in content models",

Well you don't generally get rules saying you don't have to do
something :-)

There is indeed no requirement that elements referred to in content
models be declared if they do not appear in the instance.

I'm not quite sure why this is; most likely it is to make it easier
to parametrise or subset DTDs.  For example, if you have two DTDs
which are identical except for the presence of one element, you
can put that element's declaration in a conditional section without
having to change the declarations for all the elements in which it
is allowed.

And of course, just because something isn't invalid doesn't mean a
parser can't warn about it.

-- Richard

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