CSS Styling is lost in an HTML Content Block when switching from Source mode to WYSIWYG mode in GWT editor

In order to apply styling to a resource in an HTML content block, I created a resource folder (in the user "Athabasca" collection) and added the .css file into that folder. When I edited the resource to call that file and then switched from "Source View' to WYSIWYG mode in the editor, it seemed to work - the styles were correctly applied in WYSIWYG mode. I clicked Save. When the page refreshed, the styling was lost.

Here's the page:
http://www.curriki.org/xwiki/bin/view/GWT/Editor?xpage=plain&page=Coll_Athabasca.Lesson1TheBasicSentence&mode=edit

And the css call I used:
<link rel="stylesheet" href="http://www.curriki.org/xwiki/xwiki/download/Coll_Athabasca/athabascaESLstyles/athabascaESL.css" type="text/css">

– I also used the @import method on the same files with the same results –