link tag is removed by ckeditor
As Clive said there is almost certainly no situation that CKeditor  can validly edit/create an html head element. Since link must be a child of a head element this means that CKEditor cannot create a valid link element.

That being said there is a way to add a stylesheet the the CKEditor. You will have to allow JavaScript in whatever filter you are using (so use with caution) and then insert a script like this:

&lt;script&gt;
  (function($) {
    $('head').apend("&lt;link rel='stylesheet' href='my/added/stylesheet.css'&gt;");
  }(jQuery))
&lt;/script&gt;


Or if you don't want to use jQuery:

&lt;script&gt;
  var element = document.createElement("link");
  element.setAttribute("rel", "stylesheet");
  element.setAttribute("href", "/my/added/stylesheet.css");
  document.getElementsByTagName("head")[0].appendChild(element);
&lt;/script&gt;