Participate in origin trial for Accept-Language Reduction

Accept-Language Reduction is an effort to reduce passive fingerprinting
surfaces by reducing the user’s language preferences in the
Accept-Language
header and only sending the user’s most preferred language (just one).

Beginning with the Chrome 109
Beta, we’ll open up
the origin trial
for Accept-Language Reduction enabling sites to opt into receiving the reduced
Accept-Language header. This will allow sites to discover and fix problems
before the reduced Accept-Language becomes the default behavior in Chrome in a
future release. To test the feature before it launches to stable population, be
sure to opt in and test before the release date for Chrome 109 (currently
scheduled for Jan 10th, 2023).

Check out the examples of Accept-Language header before and after the reduction below.

Accept-Language: en-GB,en;q=0.9,de;q=0.8,fr;q=0.7

Accept-Language: en-GB

Below is an overview of the origin trial and what to expect. You can share your
feedback on this change or any issues you encounter during the origin trial in
the Accept-Language Reduction GitHub
repository.

# What is Accept-Language?

The
Accept-Language
string is shared on every HTTP request and exposed in JavaScript to all
resources loaded by the browser. Currently, it contains all of the user’s
preferred languages.

# Why is Accept-Language being reduced?

Accept-Language Reduction is an effort to reduce passive fingerprinting
surfaces in Chrome.

Currently, the Accept-Language header is shared by default on every HTTP
request and exposed in JavaScript to all resources loaded by the browser. It
contains all language preferences of the user. Rather than the browser sending
the full list of languages that the user has configured in case sites wish to
provide multilingual content, we are introducing a new way for sites to indicate
multilingual content and the browser will take responsibility to do the language
negotiation and display the preferred language.

Another reason is that many sites may not be using the Accept-Language headers
for language negotiation at all (for example, one
study indicates only 7.2% of the
top 10,000 sites use Accept-Language). Chrome Incognito mode already reduced the
Accept-Language to one.

# What does this mean for web developers?

Sites that rely on Accept-Language to do language negotiation should
prepare to receive reduced Accept-Language and consider participating in the
origin trial. The reduced Accept-Language values will appear in:

• The Accept-Language HTTP request header.
• The navigator.languages JavaScript getter.

The browser will take responsibility for language negotiation to select the
user’s preferred language to send to sites. To make this happen, sites need to
add two headers
Variants
(a new header indicates sites supporting languages) Accept-Language and
Content-Language in the
response header (see detailed example below).

The reduced Accept-Language plans currently don’t include iOS and WebView, and
those platforms will continue to get users’ full list of Accept-Language.
Support for these platforms is planned for a later date.

# Origin trial for Accept-Language Reduction

An origin trial requires the participating site to provide a token in its
response which tells the browser to enable the specified trial. However, this
means that on the browser’s initial request to the site it has no way of knowing
if the site is participating in an origin trial. This means that the initial
request in a session will not send the reduced Accept-Language header.
Requests for subresources in that page, both same-origin and cross-origin, will
receive the reduced Accept-Language header. Subsequent same-origin navigations
will also receive the reduced Accept-Language header. Cross-origin navigations
will revert to sending the full header, while the cross-origin requests within the
page (such as third-party iframe requests) will still send the reduced Accept-Language
header if the top-level frame request has a valid origin trial token.

This is similar to the User-Agent reduction origin trial and if you would like
to learn more about the internal Chromium implementation, you can read more in
Implementing Reduce Accept-Language HTTP header.

# Participate in the Accept-Language Reduction origin trial

You can read further guidance in
Getting started with Chrome’s origin trials,
but the essential steps are shown below.

# Step 1

To register for the origin trial and get a token for your domains,
visit the
Trial for Accept-Language Reduction
page.

# Step 2

Update your HTTP response headers:

1. Add Origin-Trial: <ORIGIN TRIAL TOKEN> to your HTTP response
   header, where <ORIGIN TRIAL TOKEN> contains the token you got when
   registering for the origin trial.
2. Add Content-Language to your HTTP response header to indicate
   the language(s) intended for the audience.
3. Add Variants to your HTTP response header to indicate sites
   supported languages.
4. Add Vary: Accept-Language to your HTTP response
   to create a cache key for content negotiation.
5. Setting those headers will only trigger browser language
   negotiation (a potential restart for the initial request) for the given
   origin. To make sites display the correct language representation for
   the users, you also need to update sites sending the content based on
   the user’s Accept-Language header (see example below).

# Step 3

Load your website in Chrome M109 Beta (or later) and start receiving the
reduced Accept-Language string.

Submit any issues or feedback to the Accept-Language Reduction GitHub
repository.

# Demo

For a demonstration of a multilingual site which opted-in the origin trial
(along with the source code) see
https://reduce-accept-language.glitch.me/.

For a demonstration of opt-in and opt-out the origin trial (along with the
source code) see
https://reduce-accept-language-ot.glitch.me/.

For example, example.com supports ja (Japanese) and en (English). A
request may be:

GET / HTTP/1.1
Host: example.com
Accept-Language: en

The site knows the user prefers content in English based on the user’s
accept-language. The response headers may include:

HTTP/1.1 200 OK
Content-Language: en
Variants: Accept-Language=(en ja)
Origin-Token: a-valid-token

If user has a preference for Japanese language content, the request would be:

GET / HTTP/1.1
Host: example.com
Accept-Language: ja

In this case, the site responds with headers for Japanese content:

HTTP/1.1 200 OK
Content-Language: ja
Variants: Accept-Language=(en ja)
Origin-Token: a-valid-token

On the server-side, the site may be looking for specific language support but
falling back to a default if no support is detected:

if(accept_language == 'ja') {
    res.response('ja_page')
}
else {
   res.response('en_page')
}

In the example above, example.com responds either en or ja based on the
Accept-Language value, defaulting to en if none match. In this case, the
site can also provide redirects to corresponding language pages either /en or
/ja based on the Accept-Language value. See detailed examples related to
redirects on
the implementation doc.

# Third-party origin trial support

We currently don’t support registering your domains as a
third-party for the trial.
If you operate a service that is implemented as a subresource across origins
(like ad serving or analytics), you will only receive the reduced
Accept-Language header if the top-level site is participating in the origin
trial.

# Validate that the origin trial is working

The guide to
troubleshooting Chrome’s origin trials
provides a full checklist for ensuring your token is correctly configured.

You configure multiple languages and their priority from
chrome://settings/languages or Settings → Languages. Consider selecting a
language your site does not support and moving that to the top of the list
to ensure that the additional renegotiation is triggered.

The initial response’s headers containing the origin trial token should look
like:

Subsequent request headers containing the reduced Accept-Language look like:

# Stop participating in the origin trial

At any given point in time during the trial, you can stop participating and
receive the full list of user’s Accept-Language. To stop participating:

1. Remove the Origin-Trial header for the Accept-Language Reduction
   trial from your HTTP response.
2. Remove Variants header which is added to opt-in the
   origin trial in your HTTP response if you are not interested in sending
   this header. You can also use Variants with an empty value to accomplish
   this.
3. Remove Content-Language header which is added to
   opt-in the origin trial in your HTTP response if you are not interested in
   sending this header.

# Origin trial duration

The Accept-Language Reduction origin trial will run for at least six months,
which corresponds to about six Chrome milestones. The origin trial will appear
in M109 and end with M114 (it means the last Chrome release that has the trial
available). At that point, Chrome will evaluate the feedback from the origin
trial before proceeding with sending the reduced Accept-Language string in a
phased manner: first reduce the Accept-Language HTTP header and then reduce the
JS interface. If a site needs more time to test they can opt into a subsequent
deprecation origin trial, which would allow them to access the full
Accept-Language string for at least another six months. We will publish more
details on the deprecation trial when it’s ready.

# Share feedback

Submit any issues or feedback to the Accept-Language Reduction GitHub
repository.