If you actually want a preview of the live page, you'll have to use an iframe. If you just want an image preview, Fulvio's suggestion will work, but won't show a "live" preview (i.e., no animations that you would normally see, if the user is logged in to a page, you will only see the front page, etc). It is possible to actually scale the contents of an iframe so that it's a thumbnail, thus achieving the effect you want. For example:
<html>
<head>
<!--[if IE]>
<style>
#frame {
zoom: 0.2;
}
</style>
<[endif]-->
<style>
#frame {
width: 800px;
height: 520px;
border: none;
-moz-transform: scale(0.2);
-moz-transform-origin: 0 0;
-o-transform: scale(0.2);
-o-transform-origin: 0 0;
-webkit-transform: scale(0.2);
-webkit-transform-origin: 0 0;
}
</style>
</head>
<body>
<iframe id="frame" src="http://www.google.ca">
</iframe>
</body>
</html>
Have fun :)
Copy and paste into your browser's URL bar to preview:
data:text/html,<html><head><!--[if IE]><style>#frame{zoom:0.2;}</style><[endif]--><style>#frame{width:800px;height:520px;border:none;-moz-transform:scale(0.2);-moz-transform-origin:0 0;-o-transform:scale(0.2);-o-transform-origin:0 0;-webkit-transform:scale(0.2);-webkit-transform-origin:0 0;}</style></head><body><iframe id="frame" src="http://www.google.ca"></iframe></body></html>