The <amp-iframe> spec, and the validation code, reference the 600px/75% rule. However there is no description of what width that applies to. For example 600px down on a iPhone 4 is a completely different proposition to the same content on an iPad pro or a desktop. What width does the CDN use when validating? Do publishers have to guess based on the user agent what width they are dealing with and adjust content? Always use worst case?