Google announced that it will begin penalizing pages with intrusive interstitials — popup ads — beginning in January 2017:
“Pages that show intrusive interstitials provide a poorer experience to users than other pages where content is immediately accessible. This can be problematic on mobile devices where screens are often smaller. To improve the mobile search experience, after January 10, 2017, pages where content is not easily accessible to a user on the transition from the mobile search results may not rank as highly.”
The will be implemented as part of the Google’s continual push for mobile accessibility.
Not all interstitials will be penalized as Google identified three types that it considers intrusive: popups that cover the main content after the user navigates to a page from the search results (either immediately or while the user is browsing the page), standalone popups that must be dismissed before accessing content, and the use of a layout where the popup mimics the above-the-fold content but original content has been inlined underneath the fold.
While those listed may seem to cover nearly all popups, Google will make a few allowances for those that manage legal obligations: cookie usage or age verification, login dialogs for pages that are not indexed, and “banners that use a reasonable amount of screen space and are easily dismissible.” Google did not specify a size that is acceptable for popups but identified the app install banners provided by Safari and Chrome as examples of interstitials that use a reasonable amount of space.