Client Hints Infrastructure

Draft Community Group Report,

This version:
https://wicg.github.io/client-hints-infrastructure
Issue Tracking:
GitHub
Inline In Spec
Editor:
Yoav Weiss (Google)

Copyright © 2024 the Contributors to the Client Hints Infrastructure Specification, published by the Web Platform Incubator Community Group under the W3C Community Contributor License Agreement (CLA). A human-readable summary is available.

Abstract

Specification of the Client Hints infrastructure and its integration with Fetch and HTML

Status of this document

This specification was published by the Web Platform Incubator Community Group. It is not a W3C Standard nor is it on the W3C Standards Track. Please note that under the W3C Community Contributor License Agreement (CLA) there is a limited opt-out and other conditions apply. Learn more about W3C Community and Business Groups.

1. Introduction

Client Hints is collection of HTTP and user-agent features that enables privacy-preserving, proactive content negotiation with an explicit third-party delegation mechanism:

The goal of Client Hints is to reduce passive fingerprinting on the web while enabling scalable and privacy preserving content adaptation between client and server, via a standardized set of content negotiation primitives at the HTTP and user agent levels.

2. Infrastructure definition

The specification of the Client Hints infrastructure is divided between the following specifications and proposals:

3. Environment settings object processing

3.1. Client hints set

A client hints set is a set of client hints tokens.

3.2. Accept-CH cache

An Accept-CH cache is owned by the user agent and is an ordered map, keyed on origin (an origin), with a value of client hints set (a client hints set).

The Accept-CH cache can effectively act as an alternative cookie store, since sites can use each of the hints as a bit set on the client, and that information will be communicated to them on every request. As such, a user agent MUST evict that cache whenever the user clears their cookies or when session cookies expire. A user agent also MUST clear the Accept-CH according to the rules of the Clear-Site-Data Header.

A site can clear the browser’s Accept-CH cache for its origin by sending an empty Accept-CH header in a response. This sets the origin’s client hints set to an empty set.

There MAY be multiple Accept-CH headers per-response and sf-lists can be split across lines as long as each line contains at least one token.

Note: As the cache can only be modified by the top-level frame, it is considered to be partitioned.

When asked to update the client hints set from cache given a settingsObject:

  1. Let hintSet be an empty ordered set.
  2. Let originMatchingEntries be the entries in the Accept-CH cache whose origin is same origin with settingsObject’s origin.
  3. For each entry in originMatchingEntries, for each token in entry’s client hints set, append the token to hintSet.
  4. Let frameHintSet be the ordered set of hints requested by the ACCEPT_CH Frame (if any).
  5. For each hint in frameHintSet append the hint to hintSet.
  6. For each hint in hintSet, append hint to settingsObject’s client hints set.

When asked to create or override the cached client hints set given a settingsObject and response:

  1. If settingsObject is a non-secure context, abort these steps.
  2. Let browsingContext be settingsObject’s global object's browsing context.
  3. If the top-level browsing context does not equal browsingContext, abort these steps.
  4. If response’s Accept-CH header is not present, abort these steps.
  5. Let hintSet be an empty ordered set.
  6. For each hint in the result of parsing Accept-CH according to the header parsing rules, as a field-name:
    1. If hint is a client hints token add it to hintSet.
  7. Let origin be response’s origin.
  8. Set Accept-CH cache[origin] to hintSet.

3.3. Critical-CH

If an origin is loaded and the server sets an Accept-CH header that lists hints not already in the current Accept-CH cache that means only subsiquent loads of that origin will include the hints. If it’s critical that every load (including the first) has the requested Client Hints, then the server can set a Critical-CH header to request a restart. The Critical-CH header itself does not modify the Accept-CH cache.

A restart will only occur when a hint in the Accept-CH header is both not in the Accept-CH cache and in the Critical-CH header. If hints listed in the Critical-CH header are already in the Accept-CH cache no restart is needed as they were sent. If hints listed in the Critical-CH header are not in the Accept-CH header a restart would not result in the hints being included anyway.

The restart retries the entire navigation (including any prior redirects).

There MAY be multiple Critical-CH headers per-response and sf-lists can be split across lines as long as each line contains at least one token.

When asked if the user agent should restart loading the page for critical client hints given a settingsObject and response:

  1. If settingsObject is a non-secure context, abort these steps.
  2. Let browsingContext be settingsObject’s global object's browsing context.
  3. If the top-level browsing context does not equal browsingContext, abort these steps.
  4. If response’s Accept-CH header is not present, abort these steps.
  5. If response’s Critical-CH header is not present, abort these steps.
  6. Let missingHintSet be an empty ordered set.
  7. For each hint in the result of parsing Accept-CH according to the header parsing rules, as a field-name:
    1. If hint is a client hints token add it to missingHintSet.
  8. Let origin be response’s origin.
  9. For each hint in Accept-CH cache[origin]:
    1. Remove hint from missingHintSet if present.
  10. For each hint in the result of parsing Critical-CH according to the header parsing rules, as a field-name:
    1. If hint is in missingHintSet return true.
  11. Return false.

3.4. Delegate-CH

There MAY be multiple Delegate-CH tags per-document and this algorithm is run once for each in the order of the appearance of the tags.

Note: This metadata appends client hints tokens to the environment settings object's client hints set. It does not add those hints to the Accept-CH cache.

  1. Let metaElement be the meta element.
  2. If any link, style, or script elements have begun to execute, then return.
  3. If metaElement has no http-equiv attribute, or if that attribute’s value is not delegate-ch, then return.
  4. If metaElement has no content attribute, or if that attribute’s value is the empty string, then return.
  5. Let settingsObject be metaElement’s relevant settings object.
  6. If settingsObject is a non-secure context, abort these steps.
  7. Let browsingContext be settingsObject’s global object's browsing context.
  8. If the top-level browsing context does not equal browsingContext, abort these steps.
  9. Let permissionsPolicy be metaElement’s node document’s permissions policy.
  10. Let delegateCHValue be the value of metaElement’s content attribute.
  11. Let policyDirective be the result of running Permissions Policy § 9.3 Parse policy directive on delegateCHValue and the origin of the metaElement’s node document.
  12. For each feature->allowList of policyDirective:
    1. If feature is not a client hints token, then continue.
    2. If permissionsPolicy[feature]'s permissions policy is *, then continue.
    3. Let filteredAllowList be an empty ordered set.
    4. For each item in allowList:
      1. If item is origin then Append item to filteredAllowList.
    5. Append feature to settingsObject’s client hints set and Append filteredAllowList to permissionsPolicy[feature]'s permissions policy.

Clarify detection of link, style, or script element execution. [Issue #110]

3.5. Interaction with ACCEPT_CH frame

The ACCEPT_CH frame is a way for Client Hints to be requested by a server during the TLS handshake of HTTP/2 and HTTP/3. The requested hints are not persisted in the Accept-CH cache, but will be retrieved and merged when update the client hints set from cache is run. The full processing model is defined in Client Hints Reliability.

There MUST be only one ACCEPT_CH frames per-connection. Additional frames will be ignored.

4. Integration with HTML

This specification integrates with the [HTML] specification by patching the algorithms below:

Add a new field to navigable:

At populating a session history entry, in step 6 after substep 7 insert the following:

  1. Let shouldRestartForCriticalClientHints be false.
  2. If navigable’s Critical-CH restart time is 0:
    1. Let shouldRestartForCriticalClientHints be the result of running should restart page for critical client hints with the relevant settings object and response.
  3. Run create or override the cached client hints set with the relevant settings object and response as inputs.
  4. If shouldRestartForCriticalClientHints then:
    1. Set navigable’s Critical-CH restart time to the current high resolution time.
    2. Restart the initial navigation (before any redirects).
  5. If navigable’s Critical-CH restart time is not 0:
    1. Set navigationParams’s Critical-CH restart time to be navigable’s Critical-CH restart time.

Clarify how "Restart the initial navigation (before any redirects)" integrates with the HTML spec. [Issue #154]

At navigation params, append the following:

Critical-CH restart time
a DOMHighResTimeStamp used for creating the navigation timing entry for the new Document. This records the time the navigation was restarted to ensure required Client Hint headers will be sent.

4.3. Service Worker initialization

At set up a worker environment settings object, after step 6, add the following step:
  1. If worker global scope implements ServiceWorkerGlobalScope, then set settings object’s client hints set to be a clone of outside settings’ client hints set.

4.4. Standard metadata names

For the section standard metadata names, add a subsection named delegate-ch with the outlined explanation.

4.5. Extending environment settings object

An environment settings object has a client hints set: a client hints set, initially the empty set, used for fetches performed using the environment settings object as a request client.

5. Request processing

When asked to append client hints to request with settingsObject and request as input, run the following steps:

  1. If request is a non-subresource request for a "sharedworker" or "worker" destination, exit without appending any hints to the header list.
  2. Let hintSet be an empty client hints set.
  3. Run update the client hints set from cache with settingsObject.
  4. For each client hints token lowEntropyHint in the registry’s low entropy hint table, append lowEntropyHint to hintSet.
  5. If request’s client is not null, then for each client hints token requestHint in settingsObject’s client hints set, append requestHint to hintSet.
  6. For each hintName in hintSet:
    1. If request is not a navigation request for a "document" destination:
      1. Let requestPermitsHint be the result of running Permissions Policy § 9.13 Should request be allowed to use feature? given request and hintName’s associated feature in § 7.2 Policy-controlled features.
      2. If requestPermitsHint is false, then continue to next hintName.
    2. If the user agent decides, in an implementation-defined way (see § 8 Security and Privacy considerations), to omit this hint then continue.
    3. Let value be the result of running find client hint value with hintName.
    4. If the user agent decides, in an implementation-defined way (see § 8 Security and Privacy considerations), to modify value then do so.
    5. append hintName/value to the header list.

When asked to remove client hints from redirect if needed with request as input, run the following steps:

  1. For each hintToken in the list of client hints tokens:

    1. If request’s header list contains hintToken, then remove hintToken from request’s header list.

6. Integration with Fetch

This specification integrates with the [FETCH] specification by patching the algorithms below:

In HTTP-network-or-cache fetch, within step 8, after substep 23, run append client hints to request with the relevant settings object and request as input.

In HTTP-redirect fetch, after step 11, run remove client hints from redirect if needed with request as input.

In forbidden request-header, to the list of headers within step 1, add Save-Data, DPR, Device-Memory, Width, and Viewport-Width.

7. Feature Registry

Note: This section contains feature-specific definitions. New features that rely on the Client Hints infrastructure need to add their respective definitions to this registry. User Agents can implement some of those features without implementing others.

7.1. Client hints token

A client hints token is a byte-lowercase representation of one of Save-Data, Sec-CH-DPR, Sec-CH-Width, Sec-CH-Viewport-Width, Sec-CH-Viewport-Height, Sec-CH-Device-Memory, Sec-CH-RTT, Sec-CH-Downlink, Sec-CH-ECT, Sec-CH-Prefers-Color-Scheme, Sec-CH-Prefers-Reduced-Motion, Sec-CH-UA, Sec-CH-UA-Arch, Sec-CH-UA-Bitness, Sec-CH-UA-Full-Version, Sec-CH-UA-Full-Version-List, Sec-CH-UA-Mobile, Sec-CH-UA-Model, Sec-CH-UA-Platform, Sec-CH-UA-Platform-Version, or Sec-CH-UA-WoW64.

Note: A client hints token will also match the request header sent by the user agent when appropriate (as determined by the request processing algorithm).

7.2. Policy-controlled features

This document defines policy-controlled client hints features, the following policy-controlled features:

Should we tie low-entropy-ness to allowlists, generally?

7.3. Low-entropy hint table

The low-entropy hint table below defines hints that are safe to send by default due to their low amounts of entropy.

A high-entropy client hint is a client hint that is not in the low-entropy hint table.

Name Value
Save-Data a suitable Save-Data value
Sec-CH-UA a suitable UA value
Sec-CH-UA-Mobile a suitable Mobile value
Sec-CH-UA-Platform a suitable Platform value

Note: If the value transmitted by Save-Data is the empty string, the header will be omitted entirely. This is done to reduce redundant header information sent by default.

7.4. Find client hint value

When asked to find client hint value, given hint as input, switch on hint and return the result:

Save-Data
a suitable Save-Data value
DPR
a suitable DPR value
Viewport-Width
a suitable Viewport-Width value
Viewport-Height
a suitable Viewport-Height value
Width
a suitable Width value
Device-Memory
a suitable Device-Memory value
RTT
a suitable RTT value
Downlink
a suitable Downlink value
ECT
a suitable ECT value
Prefers-Color-Scheme
a suitable color theme value
Prefers-Reduced-Motion
a suitable Reduced-Motion value
UA
a suitable UA value
UA-Arch
a suitable Arch value
UA-Bitness
a suitable Bitness value
UA-Full-Version
a suitable Full-Version value
UA-Full-Version-List
a suitable Full-Version-List value
UA-Mobile
a suitable Mobile value
UA-Model
a suitable Model value
UA-Platform
a suitable Platform value
UA-Platform-Version
a suitable Platform-Version value
UA-WoW64
a suitable WoW64 value

Links for image features are broken, need to actually define that and link to them.

8. Security and Privacy considerations

This specification exposes information regarding the user’s preferences and agent, which can be used as an active fingerprinting vector. User agents implementing this specification need to be aware of that, and take that into consideration when deciding whether to implement specific hints, modify their returned values for a given hint, or omit the hint entirely.

For example, the user might have a site specific setting to override or disable specific client hints to reduce the potential for fingerprinting.

9. Terms

The following terms are defined in the HTTP specifications: field-name

Conformance

Document conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

This is an example of an informative example.

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[CLEAR-SITE-DATA]
Mike West. Clear Site Data. URL: https://w3c.github.io/webappsec-clear-site-data/
[FETCH]
Anne van Kesteren. Fetch Standard. Living Standard. URL: https://fetch.spec.whatwg.org/
[HR-TIME-3]
Yoav Weiss. High Resolution Time. URL: https://w3c.github.io/hr-time/
[HTML]
Anne van Kesteren; et al. HTML Standard. Living Standard. URL: https://html.spec.whatwg.org/multipage/
[INFRA]
Anne van Kesteren; Domenic Denicola. Infra Standard. Living Standard. URL: https://infra.spec.whatwg.org/
[NAVIGATION-TIMING-2]
Yoav Weiss; Noam Rosenthal. Navigation Timing Level 2. URL: https://w3c.github.io/navigation-timing/
[PERMISSIONS-POLICY-1]
Ian Clelland. Permissions Policy. URL: https://w3c.github.io/webappsec-permissions-policy/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://datatracker.ietf.org/doc/html/rfc2119
[RFC8942]
I. Grigorik; Y. Weiss. HTTP Client Hints. February 2021. Experimental. URL: https://www.rfc-editor.org/rfc/rfc8942
[SERVICE-WORKERS]
Jake Archibald; Marijn Kruisselbrink. Service Workers. URL: https://w3c.github.io/ServiceWorker/
[URL]
Anne van Kesteren. URL Standard. Living Standard. URL: https://url.spec.whatwg.org/

Informative References

[CLIENT-HINT-RELIABILITY]
David Benjamin. Client Hint Reliability. URL: https://datatracker.ietf.org/doc/html/draft-davidben-http-client-hint-reliability
[PERMISSIONS-POLICY]
Ian Clelland. Permissions Policy. URL: https://w3c.github.io/webappsec-permissions-policy/

Issues Index

Clarify detection of link, style, or script element execution. [Issue #110] ↵
Clarify how "Restart the initial navigation (before any redirects)" integrates with the HTML spec. [Issue #154] ↵
Should we tie low-entropy-ness to allowlists, generally? ↵
Links for image features are broken, need to actually define that and link to them. ↵