Your code works fine. You just aren’t using jsFiddle’s API correctly. Check the docs for /echo/html/
(http://doc.jsfiddle.net/use/echo.html#html):
URL: /echo/html/
Data has to be provided via POST
So, you need to update your AJAX call to use POST. Also the trailing slash is needed.
$(function () {
$.ajax({
url: "/echo/html/",
type: "post",
dataType: "html",
success: function (data) {
$('#data').text(data);
$('#wtf').html($(data).find('#link').text());
},
data: {
html: '<!DOCTYPE html><head><title><\/title><link href="../css/popup.css" rel="stylesheet" /><\/head><body><ul><li><a id="link">content<\/a><\/li><\/ul><\/body><\/html>'
}
});
});