addStylesheetRule

function addRule( selector, property, sheetindex, ruleindex ) {
	if( sheetindex == undefined ) sheetindex = 0;
	var sheet = document.styleSheets[ sheetindex ];
 
	if( sheet.addRule ) { //IE
		if( ruleindex == undefined ) ruleindex = sheet.rules.length;
		sheet.addRule( selector, "{" + property + "}", ruleindex );
		return ruleindex;
	}
	else if( sheet.insertRule ) { //Mozilla
		if( ruleindex == undefined ) ruleindex = sheet.cssRules.length;
		return sheet.insertRule( selector + "{" + property + "}", ruleindex );
	}
 
	return null;
}