I thought I would share this answer for others, since it took me some time to figure out:
I am using jsbin and jsfiddle for testing web pages. I have many javascript and CSS files (as well as images referenced from the CSS files). I am using DropBox to store the folders and files with Javascript and CSS. The issue is that Dropbox changed their policy recently with Public Folders. If you share a link to a CSS file from a non-public folder you will NOT get the CSS (text/css) mime type, but a nicely formated text/html. In JSBIn (firebug) you will see and error like:
The stylesheet https://www.dropbox.com/s/y9ms5jr6zqdans2/leaflet.css was not loaded because its MIME type, "text/html", is not "text/css".