New iteration of the draft spec

I have uploaded a new version of the draft spec, temporarily at
    https://homepages.cwi.nl/~steven/ixml/ixml-specification.html

This version:

* requires a serialized name to be an XML name
* collects all non-syntactic requirements in the conformance section
* adds references

It doesn't yet address the action I have about whitespace in attributes.

The previous version is available at 

   ixml-specification-2021-01-28.html

Best wishes,

Steven