Updated errors draft

Hi folks,

I did a little more tinkering with the errors draft, 

  https://github.com/invisibleXML/ixml/pull/54

You can preview a formatted version at

  https://ndw.github.io/ixml/ixml-specification.html

I wrote a stylesheet to generate the errors appendix, but it has to be
pasted in by hand. DTD-validated XHTML is just a mess to process.

I realized in the course of writing some of the errors that they’re
interesting in the following way: they can’t happen in ixml. For
example, you can’t create an @hex with an invalid hex character because
the grammar won’t match it. Of course, an implementation still has to
check for it and raise an error if it occurs because it might arise in
hand-edited vxml. The error messages just seem a little odd in the spec.

We could take a different approach and produce a normative schema for
vxml documents. It would then be possible to say simple that it was an
error if the vxml was not valid. But (a) I’m not sure that would be more
useful for users, (b) it would require us to construct and agree on a
normative schema, and (c) I’m not sure implementors are going to be
inclined to do schema validation every time a vxml document is loaded.

                                        Be seeing you,
                                          norm

--
Norm Tovey-Walsh
Saxonica