You want something to show html after it's been changed by javascript - unless I've understood incorrectly, this is just a browser.
Try opening your document with your chosen browser: It will load all the html, and carry out any javascript that isn't contained within a function. You will probably only want the javascript to execute once the HTML has finished loading, so you could put a script tag at the bottom of the page
Alternatively, and perhaps better practice, try setting up an initialisation function, which is called once the page has finished loading.
<html>
<head>
<script>
function init(){
// resize page depending on pixels or something
}
</script>
</head>
<body onLoad="init()">
</body>
<script>
// javascript goes here
</script>
</html>
If you are looking for some sort of debugging, there are a few options out there, including ones within browsers.
Chrome
Firefox