HTML & XML Parsers

Any web browser must be able to parse HTML, and XML becoming more and more important. This component would provide a pair of parsers (or a generic SGML parser that can deal with both subsets) that output a consistent parse tree. This would be the foundation for the DOM work (see below).

• How to deal with old-style HTML, or HTML that does not conform to the DTD? Should we key off of the existence of a valid DOCTYPE and use a strict parser, and fall back to something based off of the current w3-parse.el code for DOCTYPE-less documents? Or always use the same heuristics to guess what the author really meant?
• Do we really need two separate parsers for HTML and XML? PSGML can parse well-formed HTML or any XML document (which is by definition well-formed, or the parser can gleefully choke it to death).
• Can PSGML be persuaded to do what we want? It seems that using the existing API (sgml-top-element, sgml-element-next, sgml-element-content) would be feasible. On the plus side, this would allow the DOM to work on arbitrary SGML documents (LinuxDoc or DocBook anyone?).
• Should the parsers be able to deal with streaming data? It would be theoretically possible to parse the document as it comes in off the network. Do we really care?