Google announced that it will begin penalizing pages with intrusive interstitials, commonly known as popup ads, beginning in 2017.

The will be implemented as part of the search engine’s continual push for mobile accessibility. Two years ago, Google added a mobile-friendly label to search results where users didn’t have to zoom to read text. After finding that 85% of all pages in the mobile search results are now mobile friendly, the label will be removed. In April 2015 Google updated its algorithm to include mobile friendliness as a ranking signal, which may have helped spur website owners to meet the criteria.

Not all interstitials will be penalized. 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 examples may seem to encompass nearly all interstitials, Google will make a few allowances for popups that manage legal obligations, such as 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.