Validating xml parser definition
The essence of why extensible markup languages are necessary is explained at Markup language (for example, see Markup language § XML) and at Standard Generalized Markup Language.
Hundreds of document formats using XML syntax have been developed, including RSS, Atom, SOAP, SVG, and XHTML.
XML is widely used in a Services Oriented Architecture (SOA).
Disparate systems communicate with each other by exchanging XML messages.
It implements the XML standard, namespaces, XML Base, XML Inclusions (XInclude) 1.0, XML Catalogs Working Draft, Canonical XML Version 1.0, W3C XML Schemas Part 2, etc. The entire library is contained in a single header file, so no building or linking is needed.
Note that the libraries listed vary in their approach.
The message exchange format is standardised as an XML schema (XSD). XML has come into common use for the interchange of data over the Internet.
IETF RFC:3023, now superseded by RFC:7303, gave rules for the construction of Internet Media Types for use when sending XML. Further guidelines for the use of XML in a networked context appear in RFC 3470, also known as IETF BCP 70, a document covering many aspects of designing and deploying an XML-based language.
Which you choose will of course depend on your purpose and requirements.
Libxml2 is a C XML parser library, with an assortment of bindings/wrappers for other languages if you don't use C (eg, C , Perl, Delphi/Pascal, Ruby, PHP, Java, Rexx, Apple Script, etc). Rapid XML is a minimal DOM-style XML parser designed for speed.
In publishing, DITA is an XML industry data standard.