Putting this in the bottom of a HTML document's body tag will enable a developer to have the body's width available for reference. It could also be used for other real-time logging, as needed.

Logger

<!--
========================================================  
LOGGER 
I go in the bottom of the body! 
https://gist.github.com/3069243
________________________________________________________
-->
<div id=logger></div>
<style>
  #logger {
    position:absolute;
    bottom:10px;
    left:10px;
    padding:20px;
    border-radius: 16px;
    background:rgba(0, 0, 0, 0.2);
    border:2px solid black;
    color:black;
  }
</style>
<script>
  var logger = document.getElementById("logger"),
      logData = function () {
        /* Getting the offsetWidth of the body is 
         * useful for doing responsive web design work.
         * Add more stuff here as needed. 
         */
        var width = document.body.offsetWidth;
        if ("textContent" in logger) {
          logger.textContent = width; // W3C
        }
        else if ("innerText" in logger) {
          logger.innerText = width;   // IE
        }          
      };
  if (logger.addEventListener) { // W3C
    window.addEventListener("resize", logData);
  } 
  else if (logger.attachEvent) { // IE
    window.attachEvent("onresize", logData);
  }
</script>
<!--
========================================================
-->