The Evolution of First-Party Sets to Related Website Sets: What Developers Need to Know

April 3, 2024

Introduction to First-Party Sets and the Transition to Related Website Sets (RWS)

In the evolving landscape of digital privacy, web browsers are phasing out third-party cookies to enhance user privacy. This shift, however, presents challenges for organizations that operate across multiple domains and rely on cookies for seamless user experiences. Enter the concept of First-Party Sets, a framework that allowed related domains to declare their association, thereby enabling certain cross-site functionalities. As the digital ecosystem continues to adapt, Google has refined this approach with the introduction of Related Website Sets (RWS).

RWS is a pivotal development in the Privacy Sandbox initiative, aiming to balance user privacy with essential web functionalities. It enables companies to formally declare relationships among their various domains, allowing browsers like Chrome to grant limited third-party cookie access for specific, user-centric purposes. This transition from First-Party Sets to RWS marks a significant step towards a more privacy-conscious web, where user journeys across different domains of the same entity can continue without the privacy concerns associated with third-party cookies. As we approach the deprecation of third-party cookies in 2024, understanding RWS becomes crucial for web developers and organizations to maintain cohesive digital experiences while respecting user privacy.

Understanding the Privacy Sandbox and the Push for Enhanced User Privacy

In the digital age, user privacy has become a paramount concern, prompting tech giants to reimagine the mechanisms underpinning user data protection. Google's Privacy Sandbox is a testament to this shift, heralding a new era where user privacy is not just an afterthought but a foundational principle. The initiative aims to develop a set of open standards to fundamentally enhance privacy on the web while supporting publishers and advertisers who rely on online advertising for revenue.

The Privacy Sandbox seeks to replace the invasive practice of third-party cookie tracking with privacy-preserving alternatives. Third-party cookies have long been the linchpin of digital advertising, enabling advertisers to track users across different websites to deliver targeted ads. However, this has often come at the expense of user privacy, with individuals' online behavior being monitored without explicit consent or understanding.

Google's solution is to phase out third-party cookies in Chrome by 2024 and introduce new technologies within the Privacy Sandbox that allow for relevant advertising and conversion measurement without revealing individual user identities. Related Website Sets (RWS), formerly known as First-Party Sets, is one such technology. It allows publishers to group their related domains, enabling them to offer a cohesive user experience—such as maintaining a single sign-on across multiple properties—without relying on third-party cookies.

The Privacy Sandbox initiative is a balancing act, striving to maintain the vitality of the web's economic model while curtailing the pervasive surveillance that has characterized the internet for years. It represents a significant step towards a more private web, where users can navigate digital spaces with confidence that their personal data is not being exploited.

The Role of Related Website Sets in a Post-Third-Party Cookie World

In the impending post-third-party cookie landscape, Related Website Sets (RWS) emerge as a pivotal framework for maintaining a coherent user experience across related web properties. As the digital ecosystem braces for the deprecation of third-party cookies, the RWS API stands as a beacon of continuity, ensuring that essential functionalities such as single sign-on, seamless navigation, and personalized settings persist without compromising user privacy.

RWS enables organizations to declare a consortium of domains as related entities, allowing browsers like Chrome to judiciously grant limited third-party cookie access for specific, user-centric purposes. This mechanism is particularly crucial for entities with multiple domain names—whether they are brand-specific, service-oriented, or geographically varied. It ensures that while users meander through the digital corridors of related sites, their experience remains uninterrupted by the need for repeated authentications or loss of context.

Moreover, RWS is not a carte blanche for unfettered data tracking. It is meticulously designed to uphold privacy by restricting the number of associated domains and mandating clear affiliation disclosures. This delicate balance between functionality and privacy is the cornerstone of RWS, making it a quintessential tool for developers navigating the new era of web privacy.

As we march towards a more privacy-conscious web, RWS provides the scaffolding for developers to rebuild user trust. By facilitating transparency and user control, while preserving the integrity of web services, RWS is poised to play a critical role in the post-third-party cookie world, ensuring that privacy and convenience do not become mutually exclusive.

Key Use Cases for Related Website Sets: From Brand Domains to Country-Specific Sites

Related Website Sets (RWS) are not merely a technical solution; it is a strategic facilitator for a range of use cases that are critical to maintaining seamless user experiences in a privacy-centric web environment. Here are some pivotal scenarios where RWS plays a vital role:

  1. Branded Domains: Companies often have multiple consumer-facing sites under different brand names. For instance, a business like Uber operates both and RWS enables a unified sign-on experience, allowing users to navigate between these services without repeated logins.
  2. App Domains: A single application may span across various domains. Microsoft, for example, uses,, and RWS ensures that users can work across these platforms without disruption, maintaining session continuity and preferences.
  3. Country-Specific Domains: Global businesses typically have localized versions of their sites, such as for the UK market. RWS allows these country-specific domains to provide localized content while recognizing the user's global identity across the company's domain network.
  4. Service and Sandbox Domains: Behind the scenes, service domains (like's and sandbox domains (such as support the main user-facing sites. RWS ensures these domains can function cohesively, sharing necessary cookies for operations like content delivery and security isolation without direct user interaction.
  5. Analytics and Measurement: RWS aids in the analytics and measurement of user journeys across owned and operated properties. This data is invaluable for businesses to enhance service quality and user experience without compromising individual privacy.

In essence, RWS is a linchpin for preserving critical functionalities that users have come to expect, while adapting to the evolving landscape of web privacy. It's a balancing act between user convenience and stringent privacy norms, one that RWS navigates with precision.

Step 5: Submitting Your Related Website Sets

Once you have defined your Related Website Sets, the submission process is the final step to formalize the relationship between your domains. This process is critical as it involves a public declaration of your domains' interconnection and ensures that the browser can recognize and respect these relationships when handling cookies.

To submit your Related Website Sets, follow these steps:

  1. Prepare the JSON File: Create a JSON file that accurately represents your Related Website Sets. This file should include the "set primary" domain and all "set members," categorized under the appropriate subsets. Use the RWS JSON generator tool provided by Chrome to ensure the correct format and validate your entries.
  2. Host the .well-known File: Upload a .well-known file to the root of each domain included in your set. This file serves as a verification mechanism to prove ownership and consent of all domains in the set.
  3. Public Submission: Submit your JSON file to the public repository designated by the browser you are targeting, such as Chrome's RWS submission platform. This step is essential for transparency and accountability, allowing for community scrutiny and feedback.
  4. Wait for Approval: After submission, there will be a review process. Browsers may implement different review periods, but the aim is to ensure that all submissions adhere to the guidelines and do not pose any privacy or security risks.
  5. Monitor and Update: Post-approval, it's crucial to monitor your Related Website Sets for any changes in domain relationships or business needs. Update your sets as necessary to maintain accurate and up-to-date information.

By carefully following these steps, developers can successfully submit their Related Website Sets, paving the way for a smoother transition in a post-third-party cookie era while maintaining essential website functionalities and user experiences.

The Increased Associated Domain Limit and Its Impact on Developers

In a pivotal update to the Related Website Sets (RWS) framework, Google has announced an increase in the associated domain limit from three to five, plus one primary domain. This change, set to take effect in Chrome 117, is a direct response to feedback from web standards participants who argued that the original limit was too restrictive for various use cases.

The expansion of the associated domain limit is a significant development for developers who manage multi-domain platforms. It provides more flexibility to include additional domains that are crucial for maintaining a cohesive user experience across an organization's digital properties. For instance, a media company with distinct domains for news, entertainment, and lifestyle content can now seamlessly integrate these under a single RWS without compromising on user functionality or privacy.

This adjustment aligns with comparable implementations by other major browsers and underscores Google's commitment to balance user privacy with the functional needs of the web ecosystem. It's important to note that RWS is not intended as an advertising solution; rather, it is designed to preserve essential user journeys and site functionalities post third-party cookie deprecation.

Developers must now re-evaluate their domain strategies, ensuring that their RWS submissions are optimized within the new five-domain parameter. This change may require a reassessment of which domains are essential for inclusion and how they are presented to users to maintain transparency and trust.

Moreover, for use cases that exceed the five associated domain limit, developers are encouraged to explore the Storage Access API (SAA), which provides an alternative method for requesting cookie access in non-RWS contexts. This dual approach of increased domain limits and leveraging SAA offers developers a robust toolkit to adapt to the evolving landscape of web privacy while preserving the integrity of user experiences.

Leveraging the Storage Access API (SAA) with Related Website Sets

The Storage Access API (SAA) plays a pivotal role in the functionality of Related Website Sets (RWS), serving as the conduit through which sites can request and gain access to cross-site cookies. This integration is particularly crucial in the context of RWS, as it enables a more granular control over cookie access, adhering to the privacy-centric ethos of the modern web.

With the SAA, sites within a Related Website Set can actively petition for cross-site cookie access. The user agent, or browser, is then tasked with the decision to automatically grant, deny, or prompt the user for this access. The SAA's integration with RWS means that browsers may choose to automatically grant cross-site access when the requesting site is a member of the same RWS as the top-level site, particularly if it belongs to a specific subset within the RWS.

This nuanced approach allows for a balance between user privacy and the seamless operation of web services that span multiple domains. For instance, a user navigating between domains of the same organization can maintain a consistent experience—such as a unified shopping cart or account sign-in—without unnecessary interruptions or repeated authentication requests.

Developers should note that the SAA requires user activation from an iframe embedding the origin requesting access. However, the RWS proposal suggests extending the API to simplify its use, potentially eliminating the need for iframe integration. This evolution would address common compatibility issues and streamline the process for developers, fostering a smoother transition to a post-third-party cookie era.

In essence, the strategic leveraging of the SAA within RWS represents a thoughtful advancement in web technology, providing developers with the tools to preserve user experience while respecting privacy boundaries.

Chrome 117: A Milestone for Related Website Sets Deployment

The release of Chrome 117 marks a pivotal moment in the deployment of Related Website Sets (RWS), a cornerstone in the Privacy Sandbox initiative. With this update, Chrome has increased the associated domain limit to five domains (plus one primary domain), a decision informed by ecosystem feedback and aligned with comparable implementations in other major browsers. This enhancement is not merely a quantitative change; it is a strategic move to balance privacy with functionality, allowing organizations to maintain a cohesive user experience across their digital properties without compromising user privacy.

For developers, Chrome 117 is a beacon of adaptability, signaling the browser's commitment to providing the tools necessary to navigate the post-third-party cookie landscape. The update offers a more flexible framework for developers to declare relationships among multiple domains, enabling critical user journeys such as seamless sign-ins, shared preferences, and consistent analytics across related sites. This is particularly crucial as we edge closer to the 2024 deadline for third-party cookie deprecation.

Moreover, Chrome 117 introduces a user prompting flow for extended use cases beyond the five associated domains, leveraging the Storage Access API (SAA) to accommodate more complex domain relationships. This feature underscores Chrome's dedication to user choice and developer needs, ensuring that even the most intricate web ecosystems can function smoothly in a privacy-first era.

As developers prepare for the future, Chrome 117 stands as a testament to the ongoing evolution of web standards, providing a robust foundation for privacy-conscious web development. It is a call to action for developers to reassess their domain strategies, embrace the new RWS framework, and contribute to a web that respects user privacy while delivering seamless digital experiences.

Addressing Potential Abuse and Ensuring Accountability with RWS

As the digital landscape evolves with the introduction of Related Website Sets (RWS), a critical concern is the potential for misuse. To safeguard against abuse and maintain accountability, RWS incorporates several robust mechanisms.

Firstly, RWS mandates a public submission process, such as through a GitHub repository. This approach ensures transparency, as all submissions are visible to users, civil society, and the web community at large. The public nature of these submissions acts as a deterrent against intentional misclassification of domain relationships.

To bolster this transparency, technical checks are in place to prevent a domain from being included in multiple RWSs, ensuring mutual exclusivity. Additionally, a .well-known file check on all domains verifies authorized submissions, and a cross-reference against the Public Suffix List confirms the validity of registrable domains within a set.

Moreover, the RWS framework sets a limit on the number of associated domains, which serves as a direct measure to curtail the scope of any potential abuse. This numeric limit is balanced to accommodate legitimate use cases while preventing the widespread tracking that RWS seeks to avoid.

In the event of suspected misuse, the public submission process allows for reporting of potentially invalid sets. Reactive enforcement measures, such as validation checks for service subset domains, can be triggered to ensure compliance with RWS guidelines.

The commitment to accountability extends to Chrome's RWS Canonical List, which will be generated based on the submission guidelines. These guidelines provide clarity to developers on the submission process and ensure that the browser's behavior aligns with the declared relationships.

In essence, RWS's abuse mitigation strategies are designed to provide a secure and accountable framework that upholds user privacy while allowing developers to maintain essential website functionalities in a post-third-party cookie era.

Next Steps for Developers: Preparing for the Implementation of Related Website Sets

As the digital landscape braces for the seismic shift away from third-party cookies, developers must pivot to embrace the new protocols. The transition to Related Website Sets (RWS) requires strategic preparation. 

Here's a roadmap for developers gearing up for the implementation of RWS:

  1. Assess Your Domain Structure: Evaluate your organization's domain architecture. Identify the primary domain and its associated domains, ensuring they align with the RWS framework. Consider the user experience and how domains interact with each other.
  2. Understand the Subset Definitions: Familiarize yourself with the different subset types—service, associated, and ccTLD variants. Determine which subset each of your domains falls into based on their function and user interaction.
  3. Review the Submission Guidelines: Chrome’s RWS submission guidelines provide a blueprint for developers. Scrutinize these guidelines to understand the criteria for a successful submission.
  4. Prepare Your Submission: Use the RWS JSON generator tool to aid in the preparation of your submission. Ensure that all domains included in your set are accurately represented and meet the necessary criteria.
  5. Test Cross-Domain Functionality: Before submission, test the cross-domain functionality to ensure seamless user experiences. Pay particular attention to authentication flows and shared preferences across domains.
  6. Monitor Updates and Feedback: Stay abreast of any updates from Chrome regarding RWS. Actively seek and incorporate feedback from the web community to refine your approach.
  7. Plan for User Prompting: For domains exceeding the associated domain limit, prepare to leverage the Storage Access API (SAA) and plan for a user prompting flow that respects user choice and privacy.

By meticulously following these steps, developers can ensure a smooth transition to RWS, maintaining functionality and user experience while adhering to the new privacy-centric web standards.

Conclusion: Evolution of First-Party Sets to Related Website Sets

In conclusion, the transition from First-Party Sets to Related Website Sets (RWS) marks a significant evolution in the way browsers handle site relationships and privacy. This change reflects a broader shift towards enhancing user privacy while still allowing organizations to maintain seamless cross-domain experiences. Developers need to be cognizant of the implications of RWS, particularly as third-party cookies are phased out in favor of more privacy-centric solutions.

The increased flexibility offered by RWS, with the ability to declare up to five associated domains, acknowledges the complex ecosystem of the modern web where brands often span multiple domains. This evolution ensures that user journeys remain uninterrupted and that essential functionalities such as single sign-on and personalized content can persist in a more privacy-aware internet landscape.

As we approach the deprecation of third-party cookies, it is imperative for developers to stay informed and adapt to these changes. Embracing RWS will be crucial for preserving critical web functionalities and ensuring a smooth transition into a new era of digital privacy. The journey from First-Party Sets to Related Website Sets is not just a rebranding, but a strategic move towards a balanced web experience that respects user privacy while catering to the operational needs of multi-domain entities.

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