resolveAppliedStyle, taken from http://stackoverflow.com/questions/7786972/how-to-determine-if-width-has-px-or/7787148 with a few variable name changes to suit my preferences.

resolveAppliedStyle.js

(function(){
    var documentElement = document.documentElement,
        matchesSelector =
                documentElement.matchesSelector ||
                documentElement.mozMatchesSelector ||
                documentElement.webkitMatchesSelector ||
                documentElement.msMatchesSelector ||
                documentElement.oMatchesSelector;

    if (!matchesSelector) {
        throw new Error("no browser support for selector matching");
    }

    function resolveAppliedStyle(element, style) {

        var styleSheets = document.styleSheets,
            numberOfStyleSheets = styleSheets.length,
            styleSheet,
            styleSheetIndex,
            rule,
            rules,
            numberOfRules,
            ruleIndex,
            appliedRule,
            appliedRules = [],
            appliedRuleIndex,
            numberOfAppliedRules,
            latestAppliedValue;

        for(styleSheetIndex = 0; styleSheetIndex < numberOfStyleSheets; ++styleSheetIndex) {
            styleSheet = styleSheets[styleSheetIndex];

            rules = styleSheet.cssRules || styleSheet.rules;

            if(!rules) {
                continue;
            }

            numberOfRules = rules.length;

            for(ruleIndex = 0; ruleIndex < numberOfRules; ++ruleIndex) {
                rule = rules[ruleIndex];

                if( matchesSelector.call( element, rule.selectorText ) ) {
                    appliedRules.push( rule );
                }

            }
        }

        numberOfAppliedRules = appliedRules.length;

        if(!numberOfAppliedRules) {
            return element.style[style];
        }

        for(appliedRuleIndex = 0; appliedRuleIndex < numberOfAppliedRules; ++appliedRuleIndex) {
            appliedRule = appliedRules[appliedRuleIndex];

            if( style in appliedRule.style ) {
                latestAppliedValue = appliedRule.style[style];
            }
        }


        return latestAppliedValue || element.style[style];
    }

    window.resolveAppliedStyle = resolveAppliedStyle;
})();

note, does not take into consideration CSS specificity

https://github.com/keeganstreet/specificity/blob/master/specificity.js