The source of any <iframe> tags entered by users will be stripped out whenever a page is displayed, unless the beginning of the source URL matches one or more of the sites in this list.
To reduce the chance of any malicious content being served from an iframe on your site, you should try to make these strings as long as possible, while still matching the beginning of all the source URLs you wish to allow.
When you add a new site on this page, the site's favicon host is guessed from the text you enter, and this host is used for the icon. You can change the favicon once you have added a new site by clicking on the 'Edit' button.