Is this a web page or an image?
encoding, html, image
Solution
The file you linked is a polyglot - a combination of languages. It can be read and understood as both an image and an HTML file. It's a simple trick. If you look at the HTML source you can see this at the top:
ÿØÿà
A quick Google shows this looks like a JPEG header. What the creator does is store the HTML in JPEG metadata, and the JPEG image data in a html comment. Pretty nifty but not magic.
To hide the JPEG header he uses CSS rules to hide the body and show only some elements:
body { visibility: hidden; }
.n { visibility: visible; position: absolute; ...... }
Also note that it isn't valid HTML, for example because the comment to hide the image data is not closed, but that browsers still happily accept and render it.
Problem
http://lcamtuf.coredump.cx/squirrel/ According to the author, This is an embedded landing page for an image. You can link to this URL and get the HTML document you are viewing right now (soon to include essential squirrel facts); or embed the exact same URL as an image on your own squirrel-themed page: `<a href="http://lcamtuf.coredump.cx/squirrel/">Click here!</a> <img src="http://lcamtuf.coredump.cx/squirrel/">` No server-side hacks involved - the magic happens in your browser. In other words, if you pop that URL into your browser it renders as a web page, but you can also use the same URL as an image source. What kind of witchcraft is at work here? (Edit: source code from the above link if that site ever goes offline.)