You could use an IFRAME and have JS control scaling, but you would only be able to reduce the text size, so it’d look a mess after a while. I suspect the site image was created using server-side methods, then scaled using JS or perhaps Flash.
moz transform should work, for Firefox at least
(please google it as I’m not allowed to post links just yet)
so the following code
<iframe style=“width:1300px; height:900px; -moz-transform:scale(0.75, 0.75)” src=“somewebpage.html/”></iframe>
should produce an iframe containing a functional somepage.html reduced by 25%