Extension languageExtension languages are, literally, additions to a programming or mark-up language that "extend" the functionality of the language. On the World Wide Web, you will most often hear about "Netscape Extensions" to HTML.
Netscape extensions are controversial in that they can be read only by Netscape browsers. Some industry analysts support the extensions on the grounds that Netscape incorporates extensions into their browsers to push for the development of standards. Others claim that Netscape incorporates new features to pressure the HTML working group to adopt their enhancement as a standard.