When built as UNITY_WEBGL, the WWW class seems to abort at zero bytes when accessing resources from a different domain, even if the necessary crossdomain.xml file is in place.
Unity receives zero bytes from the WWW object and the Developer toolbars in IE, Chrome, Firefox all report success codes of 200 on zero byte downloads for the URLs hit by the WWW classes. The same code hitting the same URLs returns actual data (more than zero bytes) when built for other forms of Unity like UNITY_WEB or UNITY_STANDALONE.