Since Jenkins usually runs over HTTPS browsers (in our case Firefox but Chrome behaves afaik the same way) will block external content pulled via HTTP due security concerns with the following error message:
locked loading mixed active content "http://yui.yahooapis.com/pure/0.3.0/pure-min.css"

The issue and reason is described further at https://blog.mozilla.org/tanvi/2013/04/10/mixed-content-blocking-enabled-in-firefox-23/

The yahooapi is also available over HTTPS, but the cert is not valid since the certificate of Akamai (and one of their URLs) is presented. Which leads to same problem as before.

thx