bibtexml.rnc|xsd|dtd is a direct representation of the BibTeX in XML, while in bibtexmlExtended.* we have added syntax for more detailed tagging, e.g. a comma separated list of authors can now be tagged individually with firstname and lastname. Strict documents are always valid loose documents, but not the other way around.
To XMLize BibTeX data, use bib2xml, the included bibtex2xml Python script, or BibTeXConverter which can be started using Java Web Start.
For the intention of not typing bibliography metadata at all, search The Collection of Computer Science Bibliographies for BibTeX formatted data, or convert MARC or RIS data exported from library databases, e.g. Library of Congress or Bibsys (NOR), using bibutils or the included experimental MARC-to-BibTeXML converter. Books for Mac OS X which autofill the rest of the fields based on sparse information or barcode scanning.
Optionally, MODS can be used as an intermediate format: Convert BibTeX (.bib) to MODS XML with bibutils, which can convert BibTeX, RIS, Endnote, and other bibliography formats to MODS XML (and back). For pretty printing of the MODS data in HTML, use CiteProc.
XSLT specs for converting BibTeX XML markup to BibTeX (LaTeX syntax), Dublin Core, MODS, RIS, DocBook, LaTeX biblist environment, or HTML using Harvard, Chicago or APA citation styles. There is also a testbed for generating HTML with links to the Amazon.com bookstore. See some examples.