If you want to use XML & XHTML forms you will need to know:
The XML standard– XML repurposes & cleans up HTML so it can be use for arbitrary knowledge domains. No prerequisites, but to use it well, you need to set up DTDs & Schemas.
XML Schema standard– Defines what a document should look like and what the data types are.
The XSLT standard– Allows you to transform XML into different XML or into non-XML like fixed width files or HTML. Requires knowing the XPath standard
XPath standard– This is like SQL WHERE clauses, except for the data is in XML. Takes getting used to because unlike SQL WHERE clauses, the data being queried is heirarchial.
The XHTML standard– This is the set of rules to be applied to a HTML standard document so that it is tidy enough to be used as XML. It requires knowing HTML4.01
HTML4.01– This standard is huge– 380+ pages.