Currently, seamless iframes work only with same-origin document because otherwise the parent document could steal information from the child document. 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>