Work in progress for an unused CSS selector javascript utility

bookmarklet.js

(function() {
	var s=document.createElement('script');
	s.setAttribute('src','https://ajax.googleapis.com/ajax/libs/jquery/1.4.4/jquery.min.js');
	if(typeof jQuery!='undefined') {
		var msg='This page already using jQuery v' + jQuery.fn.jquery;
	} else {
		document.getElementsByTagName('head')[0].appendChild(s);
		var msg='This page is now jQuerified'
	}
	
	window.setTimeout(function() {
		jQuery.noConflict();
		jQuery(document).ready(function() {
			var existing_classes = [];
			var defined_classes = [];
			var unused_classes = [];
			jQuery('body').find('*').each(function(){ 
				if(jQuery(this)[0].className) {
					jQuery.each(jQuery(this)[0].className.split(' '), function(i,v) {
						existing_classes.push(v);
					});
				}
			});
			for(i=0; i<document.styleSheets.length; i++) {
				rules = document.styleSheets[i].cssRules;
				for(var j = 0; j < rules.length; j++) {
					jQuery.each(rules[j].selectorText.split(' '), function(i,v) {
						if(v.charAt(0) == '.') {
							defined_classes.push(v.slice(1));
						}
					});
				}
			};
			jQuery.each(defined_classes, function(i,v) {
				if(jQuery.inArray(v, existing_classes) != 1) {
					unused_classes.push(v);
				}
			});
			console.log(existing_classes);
			console.log(defined_classes);
			console.log(unused_classes);
		});
	}, 2500);
})();

munkee.js

jQuery.noConflict();

jQuery(document).ready(function() {
	var existing_classes = []; // includes classes in the current document
	var defined_classes = []; // includes classes defined in the CSS
	var unused_classes = []; // classes defined in the CSS, not used in the document

	// go through each node in the DOM
	// (we ignore things outside the body element)
	// (those are quite rare)
	jQuery('body').find('*').each(function(){ 
		if(jQuery(this)[0].className) {
			// for each element, grab all classes and push them into 
			// our existing_classes array
			jQuery.each(jQuery(this)[0].className.split(" "), function(i,v) {
				existing_classes.push(v);
			});
		}
	});
	
	// go through our document stylesheets
	// extract the selector text from each rule
	for(i=0; i<document.styleSheets.length; i++) {
		rules = document.styleSheets[i].cssRules;
		for(var j = 0; j < rules.length; j++) {
			// go through each CSS rule, and split the selector text
			jQuery.each(rules[j].selectorText.split(" "), function(i,v) {
				// if the selector defines a class, add it to the defined_classes array
				// note: this is probably quite crude (and slow for large documents),
				// but works for 90% of cases
				if(v.charAt(0) == ".") {
					defined_classes.push(v.slice(1));
				}
			});
		}
	}
	
	// work some array magic
	// go through our defined classes and see if they're being used
	// if not, add them to our unused_classes array
	jQuery.each(defined_classes, function(i,v) {
		if(jQuery.inArray(v, existing_classes) != 1) {
			unused_classes.push(v);
		}
	});
	
	
	// print classes used in this document
	console.log(existing_classes);
	// print classes declared by the stylesheets
	console.log(defined_classes);
	// the money shot. print unused classes
	console.log(unused_classes);
});

Thanks for this. You just need to do a check for rules[j].selectorText for selectors that don't have it (eg: font declarations):

for(var j = 0; j < rules.length; j++) {
    if (rules[j].selectorText) {
        jQuery.each(rules[j].selectorText.split(' '), function(i,v) {
            if(v.charAt(0) == '.') {
                defined_classes.push(v.slice(1));
            }
        });
    }
}