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iframe with the object element, however object support accross browsers is generally poor. I would not risk it in this case, not least for the reason that you don't want to lose revenue because an ad doesn't display! You have two options: the first is to continue using strict doctypes and merely ignore the error about unrecognized
iframe (this will have no influence whatsoever of the browser's ability to parse the page), or to switch to transitional doctypes to remain valid.