I'd like to do RelaxNG (preferred) or XSD (if I have to) schema validation in client-side JavaScript. Any good libraries for this? Google finds jsrelaxngvalidator but I'm having trouble getting it running and it seems unmaintained. Target browsers are Firefox and Chrome only.
SmartClient can load an XSD and generate a form for editing the data, complete with validation rules and user friendly error messages:
http://www.smartclient.com/#xmlSchemaImport
You can also serialize the data that the user inputs and produce an XML document that conforms to the XSD (correctly namespaced and all that) using DataSource.xmlSerialize().
http://www.smartclient.com/docs/8.0/a/b/c/go.html#method..DataSource.xmlSerialize
This functionality is all implemented in the browser (no server component) and is all in the free OSS version (SmartClient LGPL).
Note this particular feature is part of a much larger system that may or may not fit your needs - SmartClient is primarily for large scale enterprise business applications.