http://citeomatic.com/_asdf.html
(This one only works in Firefox and Opera, not Chrome, sadly)
HTTP/1.1 200 OK Date: Mon, 19 Mar 2012 08:49:27 GMT Server: Apache Link: <_asdf.css>;rel="stylesheet";type="text/css";media="all" Content-Length: 0 Connection: close Content-Type: text/html; charset=UTF-8
With httpfox I got
html:after { padding-left: 5px; content: 'Can you view my source from Firefox or Opera?'; }
Edit: It's also possible to view the source with Firefox JSview, Web Developer addons and curl.
http://citeomatic.com/_asdf.html
(This one only works in Firefox and Opera, not Chrome, sadly)