Currently, seamless iframes work only with same-origin document because otherwise the parent document could steal information from the child document, such as how much content it contains.
There are use cases (see below) for using seamless iframes with cross-origin documents. This proposal provides a way for documents to opt into being seamless with cross-origin parents.
<html allowseamless> <body> If my parent iframe has the seamless attribute, then <a href="http://example.com">this example link</a> will navigate my parent and the iframe will autosize to the right height and width. </body>
<html allowseamless="inherit-style"> <body> Now in addition to the above, I will inherit styles from my parent. Inheriting styles requires an additional opt-in because letting my parent style me could leak more information (such as the value of form input elements) to my parent. </body>