Based off of https://gist.github.com/PaulKinlan/6284142 , but with support for media queries and dumps css to a textarea in a panel stuck to the top of the page. Only tested in Chrome, uses height of window to determine "critical path".

critcss.snippet.js

(function(){
    if(document.querySelector("#_CRIT_CSS")){return;}
    var container = document.createElement("div");
    container.id = "_CRIT_CSS";
    container.innerHTML = '<textarea cols=80 rows=20></textarea><button id="CRIT_FIND">Find Critical CSS</button><button id="CRIT_CLOSE">Exit</button>';
    container.style.position = "fixed";
    container.style.top = 0;
    container.style.left = 0;
    container.style.right = 0;
    container.style.backgroundColor = "#FFF";
    container.style.zIndex = 99999999999999;
    document.body.appendChild(container);

    container.querySelector("#CRIT_FIND").addEventListener("click",function(){
        container.querySelector("textarea").value = findCriticalCSS(window,document);
    });

    container.querySelector("#CRIT_CLOSE").addEventListener("click",function(){
        container.remove();
        container = null;
    });

    function findCriticalCSS(w, d){

    //Setup our Pseudo selector killer, view height and critical nodes
    var removePseudo = /([^\s,\:\(])\:\:?(?!not)[a-zA-Z\-]{1,}(?:\(.*?\))?/g;
        var height = w.innerHeight;
        var criticalNodes = [];

    //Go find all the critical nodes
    var walker = d.createTreeWalker(d, NodeFilter.SHOW_ELEMENT, function(node) { if(node === container){console.error("FOUND IT", node)};return node === container ? NodeFilter.FILTER_REJECT : NodeFilter.FILTER_ACCEPT; }, true);
    while(walker.nextNode()) {
        var node = walker.currentNode;
        //var rect = node.getBoundingClientRect();
        if(node.offsetTop < height) {
            criticalNodes.push(node);
        } 
    }
    console.log("Found " + criticalNodes.length + " critical nodes");

    //Grab loaded stylesheets
    var sheets = document.styleSheets;

    var outCss = Array.prototype.map.call(sheets,function(sheet){
        var rules = sheet.rules || sheet.cssRules;
        //If there are rules
        if(rules){
            return {
                sheet: sheet,
                rules: Array.prototype.map.call(rules, function(rule){ //Convert each CSSRule into a 
                    try{
                        if(rule instanceof CSSMediaRule){
                            var subRules = rule.rules || rule.cssRules;
                            var css = Array.prototype.filter.call(subRules, function(rule){
                                return criticalNodes.filter(function(e){ return e.matches(rule.selectorText.replace(removePseudo,"$1"))}).length > 0;
                            }).map(function(rule){return rule.cssText}).reduce(function(ruleCss,init){return init + "\n" + ruleCss;},"");
                            return css ? ("@media " + rule.media.mediaText + " { " + css + "}") : null;

                        }else if(rule instanceof CSSStyleRule){

                            return criticalNodes.filter(function(e){ return e.matches(rule.selectorText.replace(removePseudo,"$1"))}).length > 0 ? rule.cssText : null;

                        }else{
                            console.warn("allowing", rule);
                            return rule.cssText;
                        }
                    }catch(e){
                        console.error("Bad CSS rule", rule.selectorText);
                        throw e;
                    }
                }).filter(function(e){return e;})
            }
        }else{
            return null;
        }
    }).filter(function(cssEntry){return cssEntry && cssEntry.rules.length > 0})

    //Enable only this for debug dump
    //.map(function(e){return {href: e.sheet.href, oldsize: e.sheet.cssRules.length, rules: e.rules, css: e.rules.join("\n")} })

    //Enable this for CSS
    .map(function(cssEntry){ return cssEntry.rules.join(""); })
    .reduce(function(css,out){return out + css},"")
    
    return outCss.replace(/\n/g,"").replace(/content\: \"(.)\"/g,function(a,e){
        return "content: \"\\" + escape(e).substr(2) + "\"";
    });

}

})()