When adding certain websites to display via the HTML Widget will not display any content. When viewing the ECC logs you will see errors similar to:
com.uc4.webui.vaadin.framework.dashboard.widgets.iframe.IFrameWidgetController - can not retrieve content/response code server-side for given URL:
The reason for this behaviour is that the websites are designed to not allow for their content to be embedded.
There is no workaround for this issue at this time - this is known and is being reviewed for future versions of the ECC.