<style>

    .test {

        background-color: Red;
        width: 500px;
        border-bottom: solid 1px blue;
    }

    </style>
    <script>

        function getStyleWidth(className) {
            var classes = document.styleSheets[0].rules || document.styleSheets[0].cssRules;
            var classIndex = classes.length;
            while (classIndex--) {
                if (classes[classIndex].selectorText == className) {
                    var cssText = classes[classIndex].style.cssText;
                    var styleStartIndex = cssText.search(/width:/i); // get start index
                    var styleEndIndex = cssText.indexOf(";", styleStartIndex); // get end index
                    var widthValue = cssText.substring(styleStartIndex, styleEndIndex); // remove other styles
                    widthValue = widthValue.replace(/width:/i, "").replace(/;/, "").replace(/ /, "").replace(/px/i, ""); // strip "width:" + ";" + " " + "px"
                    return widthValue;
                }
            }

        }
        alert(getStyleWidth('.test'));


    </script>