Yes, some sites may have functionality that does not work in an iframe and since the RadHtmlPlaceholder renders all external pages into an HTML Iframe control, exceptions might occur.
You can see an exception on bing.com even if you place an IFrame over the SL application. It is also present in the w3schools test page:
Therefore it is best to show only html that you have control over in the RadHtmlPlaceholder.
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