Intent to Extend Experiment: Enhanced Canvas TextMetrics

[Florin Malita]

Contact emails

jlav...@google.com, andres...@chromium.org, fma...@chromium.org, sche...@chromium.org

Explainer

https://github.com/fserb/canvas2D/blob/master/spec/enhanced-textmetrics.md
https://github.com/Igalia/explainers/blob/main/canvas-formatted-text/text-metrics-additions.md
https://github.com/whatwg/html/issues/10677

Specification

https://github.com/whatwg/html/pull/11000

Summary

Expand the TextMetrics Canvas API to support selection rectangles, bounding box queries, and glyph cluster-based operations. This new functionality should enable complex text editing applications with accurate selection, caret positioning, and hit testing. Additionally, cluster-based rendering facilitates sophisticated text effects such as independent character animations and styling.

Blink component

Blink>Canvas

Web Feature ID

No information provided

TAG review

https://github.com/w3ctag/design-reviews/issues/1095

TAG review status

Pending

Origin Trial Name

Enhanced Canvas TextMetrics

Goals for experimentation

Partners have implemented prototypes based on the available runtime feature, with positive feedback. They are now interested in expanding the scope, and testing the API with real users. This experiment will provide valuable validation in real-world use scenarios, and should help us gauge the proposed API's fitness, ergonomics, and performance.

Chromium Trial Name

ExtendedTextMetrics

Origin Trial documentation link

https://github.com/Igalia/explainers/blob/main/canvas-formatted-text/text-metrics-additions.md

WebFeature UseCounter name

kExtendedTextMetrics

Risks

Interoperability and Compatibility

These are new features and we will get vendor agreement at WHATWG before shipping. So minimal interop risks beyond platform-specific metrics differences.

Gecko: No signal (https://github.com/mozilla/standards-positions/issues/1144)

WebKit: No signal (https://github.com/WebKit/standards-positions/issues/436)

Web developers: Positive Developers want the ability to animated glyphs independendly (such as exploding sentences) and the ability to do e.g. selection on canvas text. The proposed APIs support these use cases and more.

Other signals:

Ergonomics

None.

Activation

None.

Security

There is always a fingerprinting concern with HTML canvas. THe new features expose no novel fingerprinting surface (text metrics are already a fingerprinting concern).

WebView application risks

Does this intent deprecate or change behavior of existing APIs, such that it has potentially high risk for Android WebView-based applications?

No information provided

Reason this experiment is being extended

Flutter Web has provided positive initial feedback based on internal testing. To validate the feature at scale, they require additional time to expose it to external developers via their own experimentation infrastructure.

The goal of this extension is to gather telemetry and feedback from real-world applications, allowing us to identify and address any remaining rough edges before proceeding with a formal launch.

Ongoing technical constraints

No information provided

Debuggability

DevTools supports querying the APIs by default.

Will this feature be supported on all six Blink platforms (Windows, Mac, Linux, ChromeOS, Android, and Android WebView)?

Yes

Is this feature fully tested by web-platform-tests?

Yes
html/canvas/[element|offscreen]/text... 2d.text.measure.index-from-offset* 2d.text.measure.selection-rects* 2d.text.measure.text-clusters* https://wpt.fyi/results/html/canvas/element/text?label=master&label=experimental&aligned&q=tentative https://wpt.fyi/results/html/canvas/offscreen/text?label=master&label=experimental&aligned&q=tentative

Flag name on about://flags

Experimental Web Platform Features

Finch feature name

ExtendedTextMetrics

Requires code in //chrome?

False

Tracking bug

https://issues.chromium.org/issues/341213359

Measurement

UMA counters will be added for the new APIs.

Estimated milestones

Origin trial desktop first	144
Origin trial desktop last	149
Origin trial extension 1 end milestone	155
DevTrial on desktop	141
Origin trial Android first	144
Origin trial Android last	149
DevTrial on Android	141

Link to entry on the Chrome Platform Status

https://chromestatus.com/feature/5075532483657728?gate=5178855770947584

Links to previous Intent discussions

Intent to Prototype: https://groups.google.com/a/chromium.org/d/msgid/blink-dev/CADgYMVdqo4PBs4OkGqVncRizs8vtX4YtFLDcK%2BRxdYo_wnaRJQ%40mail.gmail.com
Ready for Trial: https://groups.google.com/a/chromium.org/g/blink-dev/c/OLCCI0ExvIk
Intent to Experiment: https://groups.google.com/a/chromium.org/d/msgid/blink-dev/CADgYMVfC-Naw8pFVaUUPMSfW-u3kC7SMdDeHJgwU5YmSHOOeeA%40mail.gmail.com

This intent message was generated by Chrome Platform Status.

[Mike Taylor]

On 5/18/26 4:11 p.m., Florin Malita wrote:

Contact emails

jlav...@google.com, andres...@chromium.org, fma...@chromium.org, sche...@chromium.org

Explainer

https://github.com/fserb/canvas2D/blob/master/spec/enhanced-textmetrics.md
https://github.com/Igalia/explainers/blob/main/canvas-formatted-text/text-metrics-additions.md
https://github.com/whatwg/html/issues/10677

Specification

https://github.com/whatwg/html/pull/11000

Summary

Expand the TextMetrics Canvas API to support selection rectangles, bounding box queries, and glyph cluster-based operations. This new functionality should enable complex text editing applications with accurate selection, caret positioning, and hit testing. Additionally, cluster-based rendering facilitates sophisticated text effects such as independent character animations and styling.

Blink component

Blink>Canvas

Web Feature ID

No information provided

TAG review

https://github.com/w3ctag/design-reviews/issues/1095

TAG review status

Pending

Origin Trial Name

Enhanced Canvas TextMetrics

Goals for experimentation

Partners have implemented prototypes based on the available runtime feature, with positive feedback. They are now interested in expanding the scope, and testing the API with real users. This experiment will provide valuable validation in real-world use scenarios, and should help us gauge the proposed API's fitness, ergonomics, and performance.

Can you also comment on progress in the following areas?

Draft spec (early draft is ok, but must be spec-like and associated with the appropriate standardization venue, or WICG)
TAG review (see exceptions)
signals requests
Outreach for feedback from the spec community
WPT tests

I see there are some tests that pass in all browsers - that suggests the test isn't working as intended?

e.g., https://wpt.fyi/results/html/canvas/offscreen/text/2d.text.measure.strokeTextCluster-font-change.tentative.html?label=master&label=experimental&aligned&q=tentative

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[Vladimir Levin]

Just to add to Mike's question here, the last activity on this HTML PR https://github.com/whatwg/html/pull/11000 seems to be from roughly a year ago. Is this still under active discussion? (Similar activity on https://github.com/whatwg/html/issues/10677)

Thanks,

Vlad 

To view this discussion visit https://groups.google.com/a/chromium.org/d/msgid/blink-dev/7942727a-4657-40c4-95c3-172fcab90271%40chromium.org.

[Florin Malita]

Thanks Vlad and Mike!

Replies inline.

The spec PR needs a couple of minor updates based on TAG feedback.

Beyond that, it is pending review and agreement on https://github.com/whatwg/html/issues/10677.  The main friction point is the proposal to significantly increase scope by exposing intermediate text shaping stages and related info.  We maintain that this is non-critical for current use cases and can be deferred.  schenney@ is engaging with Apple engineers to reach a resolution.

 

TAG review (see exceptions)

 

The TAG review offered some great feedback.  One major focus was accessibility, where the group’s position is to encourage more accessible alternatives instead of expanding canvas APIs (which are generally lacking).  We discussed several alternatives, but unfortunately none of them fulfill the low-level requirements we set to solve with TextClusters.  The upcoming HTML-in-Canvas should address some of the general Canvas accessibility issues, and is a good complement to Enhanced TextMetrics (an incremental Canvas API extension focusing on low-level text rendering capabilities).

Tangentially, TAG and WHATWG feedback also led to the implementation of language and text writing mode support for Canvas, which is tracked through separate intents.
 

signals requests

 

Apple engineers were involved in the initial discussions. schenney@ is reaching out to re-engage. 

Outreach for feedback from the spec community

 

Feedback has been collected primarily through the spec PR and associated discussions. Notably, Igalia/schenney@ have published blog posts and explainers to socialise the proposal:

https://blogs.igalia.com/schenney/canvas-text-editing/
https://github.com/Igalia/explainers/blob/main/canvas-formatted-text/text-metrics-additions.md
 

WPT tests

 

WPT tests have been merged upstream as 'tentative' pending the finalization of the spec: https://wpt.fyi/results/html/canvas/offscreen/text?label=master&label=experimental&aligned&q=tentative

Thanks, agreed, that's unexpected.  I filed a bug to investigate.
 

To view this discussion visit https://groups.google.com/a/chromium.org/d/msgid/blink-dev/CADsXd2PQiHFe0teA%2BAZ6TLOB86CMCE5h6qpVvQ8_jEL3u%3Do99w%40mail.gmail.com.