Markup language

Note: You will find API References and example usage on the provided links. 

LibYAML is a YAML 1.1 parser and emitter written in C. 
[HTML] http://pyyaml.org/wiki/LibYAML 

MJSON M's JSON parser is a small library completely written in plain ISO C which handles documents described by the JavaScript Object Notation (JSON) data interchange format. 
[HTML] http://mjson.sourceforge.net/index.html 

LibAxl (or just Axl) is an implementation of the XML 1.0 standard specification written in C 
[HTML] http://www.aspl.es/axl/ 

Libcroco is a general CSS parsing and manipulation library written in C for the GNOME project. It provides a CSS2 parser (SAC and CSSOM API), and a CSS2 selection engine. It uses Libxml2 as underlying XML platform and the GLib as a portability layer. 
[HTML] https://launchpad.net/libcroco 

source: http://www.gtkforums.com/about988.html