This feature is unavailable for AMPHTML ads.
In some cases you may wish to have a mobile ad that responds to orientation changes, with separate landscape and portrait layouts. Google Web Designer lets you handle this in two ways, using either responsive layout or an alternative page.
1. Responsive layout
If you're using responsive layout, you should create separate portrait and landscape views using media rules.
Mobile ads which allow full screen video should use responsive layout so that video playback won't be interrupted by orientation changes.
2. Alternative pages
You can create alternative orientation pages for each page by going to the Page properties section of the Properties panel, and setting the Alternative Page to the page ID of the page with the opposite orientation.
If you create a non-responsive ad using separate pages for each orientation, but later change the ad to use responsive layout, you will need to clear the Alternative page property and add separate landscape and portrait media rules for your ad to work correctly.
