Prior to Netscape 6.0, this was one of the ways into the style sheet manipulation process. It is now deprecated and should not be used in any forward looking new projects unless they really need to operate with legacy browsers. Even then, compared with the MSIE and Netscape 6.0 capabilities, this is quite limited in its scope.
This roughly corresponds to the Document.styleSheets property in MSIE and is the root of the style control mechanism in Netscape .
This is not available in MSIE and is part of the Netscape 4 JSS style support and therefore to be deprecated and not used in new projects.
Although this is supposed to be an associative array, it is really a factory class that manufactures JSSTag objects on demand. If you specify a non-existent HTML tag as a property to this object, you should get an error. Unfortunately you just get a new JSSTag object.
This whole JSS style complex is deprecated, and unless you are simply keen to amuse yourself examining it and doing some experiments, it is of little practical use.
Netscape 6.0 provides a much more capable and portable interface by means of the Style object and removes all the JSS functionality.