Fenced Frames

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Google's Fenced Frames loads ads in a separate container that dissociates information within the container from the parent page.[1]

Fenced Frames are designed to address short comings in other Privacy Sandbox proposals, such as providing support for:

  • Interest Based Advertising
  • Conversion Lift measurement studies
  • Granting unpartitioned storage access without impairing user experience via a permission prompt
  • Cross-site embedded content

Fenced Frames will allow read-only access to cross-site information (e.g., audience segments, membership within an A/B experiment, access to cross-site conversion lift measurement). The Fenced Frame will only have network access beyond the web client, when initiated by a user (e.g., a click).

Chrome recommends caching embedded content to eliminate content access requests at run-time, which thus requires all permutations of personalization logic to be pre-rendered and available to local storage. Chrome calls this pre-rendered content "web bundles".

Chrome opened the door to communication toward "trusted servers" as an alternative to "web bundles", but did not elaborate on the requirements of such servers.

Note: Google has made no commitment that it will rely only on Fenced Frames to monetize its own web properties.


By removing communication between the container and the parent page, publishers lose monitoring and control over this content.

Advertiser also loose the ability to know where, when and at what precise cost his ads are rendered

Fenced Frames suggests that Chrome will allow people choice to store information in their browser that can be used across different sites. "On user activation / [Fenced Frame will have] Full network access, read/write unpartitioned storage (if requested [from the user)"[2] However, a per ad slot pop-up dialog seems like a poor user experience.

If the parent page cannot know or communicate information into Fenced Frame, it is unclear how resizing, scrolling or other user actions in the parent page will avoid causing a poor user experience with the content within the frame.

Regulator Perspectives

The UK CMA noted (5.43) that should Google impair the ability of publishers and their marketer customers from understanding what content is being delivered where, to which audiences at which time, similar to other Privacy Sandbox proposals, Google's Fenced Frame proposal would:

  1. "lead to brand safety concerns,"
  2. impair "the ability of publishers to control, measure, and optimise content on their website,"
  3. lower the value marketers would ascribe to publisher inventory, given the same in real-time control, measurement and optimization impairments to "the advertiser from knowing on which publisher inventory its ad content is being placed," and
  4. given Google's extensive owned and operated properties would give it an advantage over rivals, on account of Google continuing to have access to granular real-time feedback information and unimpeded insight into the content on their pages.[3]