You have already used the rel attribute in the link element which points to your external stylesheet:

<link rel="stylesheet" type="text/css" href="default.css">

The rel attribute can be used inside any link or a element and describes the relation of the target to the current page.  The naming of those elements is unfortunate but this might explain:

The use in link elements will come up later in the CSS section but can also be used in some of the ways explained below for a elements.  Unless the browser knows what to do about the rel it will be ignored.  Ones you might want to use in a elements include:

 

https://www.w3.org/TR/html5/links.html#linkTypes

http://www.yourwebskills.com/sitestructure.php