deluge-web requires Flash

From /usr/share/pyshared/deluge/ui/web/js/ext-all-debug.js (provided by package deluge-web):

/**
 * Sets the url for installing flash if it doesn't exist. This should be set to a local resource.
 * @static
 * @type String
 */
Ext.FlashComponent.EXPRESS_INSTALL_URL = 'http:/' + '/swfobject.googlecode.com/svn/trunk/swfobject/expressInstall.swf';

...expressInstall.swf (MIT Licensed) in turn auto-downloads a Shockwave Flash file from Adobe (non-free).