// example: 
// phantomjs getComputedStyle.js 'http://family.baidu.com/' '.item:last-child' 'font-size'

var webPage = require('webpage');
var system = require('system');

var page = webPage.create();
page.settings.loadImages = false;

var args = system.args;
if (args.length < 4) {
  console.log('usage: phantomjs getComputedStyle.js url elementId styleName');
  phantom.exit();
}
var url = args[1];
var elementId = args[2];
var styleName = args[3];

page.open(url, function(status) {
  var styleValue = page.evaluate(function(elementId, styleName) {
    var element = document.querySelector(elementId);
    return window.getComputedStyle(element).getPropertyValue(styleName);
  }, elementId, styleName);
  console.log(styleValue);
  phantom.exit();
});