Martin, thank you very, very much for looking at this.  Happy to
iterate.  I'll turn it around as fast as I can, probably tomorrow.

- Node.nodeValue isn't readonly; setting it raises an exception only
if the *node* is readonly.  DOM Level 2 clarifies: "When it is defined
to be null, setting it has no effect."

http://www.w3.org/TR/REC-DOM-Level-1/level-one-core.html#ID-1950641247
http://www.w3.org/TR/DOM-Level-2-Core/core.html#ID-F68D080

- I agree about moving the name-checking code out of minidom.
"xml.names" sounds fine.  Should it be official and documented?

  xml.names.is_name(name, [xmlVersion="1.0"]) -> bool
    Returns True if name matches the Name construction in the
    specified version of XML.  Raises ValueError if xmlVersion is not
    a known version of XML (currently either "1.0" or "1.1").

- Regarding xmlVersion = "1.1":  I don't think writing "fully
normalized" XML is required.  It's only a "SHOULD" in XML 1.1:

"All XML parsed entities (including document entities) SHOULD be fully
normalized [...]  However, a document is still well-formed even if it
is not fully normalized. XML processors SHOULD provide a user option
to verify that the document being processed is in fully normalized
form, and report to the application whether it is or not."

http://www.w3.org/TR/xml11/#sec-normalization-checking

DOM Level 3 provides a "normalize-characters" option.  Implementations
are not required to support it:

http://www.w3.org/TR/2004/REC-DOM-Level-3-LS-20040407/load-save.html#parameter-normalize-characters

I want to add normalization to writexml() and toxml(), but I hope it
can wait until after this patch lands.