If an iframing site is ever likely to be visited by users already logged in to the providing site, Themeless can disable any concrete5 edit bar and panels from appearing on the Themeless page, so they don't spoil the effect on the iframing site.
Themeless provides a dashboard page at Pages&Themes > Themes > Themeless with an option to disable the core menu bar, dashboard panel and other core attachments to the page. When checked this will keep Themeless pages minimal and free from any visible core influence, so users logged in to the Themeless providing site will not see any of these concrete5 controls when visiting a page on another iste that embeds the Themeless page in an iframe.
(note: when a user is logged in, some core assets may still be loaded behind the scenes.).