I don't consider it a showstopper for the experiment, but whenever we
add or change or improve timing functions or functionality we need to
consider if we have made timing attacks possible or simpler. Please
highlight that risk when you are in contact with privacy and security
reviewers.
/Daniel
On 2026-08-05 17:08, Chris Harrelson wrote:
Please provide an end milestone for this trial. (Note: please take
into account that releases are switching to 2-week).
On Tue, Aug 4, 2026 at 3:30 PM Chromestatus
<[email protected]> wrote:
*Contact emails*
[email protected], [email protected]
*Explainer*
https://github.com/WICG/js-self-profiling/blob/main/markers.md
*Specification*
https://github.com/WICG/js-self-profiling/pull/89
*Design docs*
https://github.com/WICG/js-self-profiling/blob/main/markers.md
https://github.com/WICG/js-self-profiling/pull/89
*Summary*
The JavaScript Self-Profiling API lets a web application sample
its own call stacks to measure performance on real user devices.
This feature adds an optional marker field to each captured sample
that identifies the type of browser activity running when the
sample was taken: script, gc, style, layout, paint, or other. A
trace normally shows gaps between stacks that cannot be
interpreted, markers let developers attribute that time to browser
work happening outside their JavaScript, for example
distinguishing script execution from style recalculation, layout,
or a garbage collection pause, making slow traces easier to
analyze and optimize.
*Blink component*
Blink>PerformanceAPIs
<https://issues.chromium.org/issues?q=customfield1222907:%22Blink%3EPerformanceAPIs%22>
*Web Feature ID*
4217 <https://webstatus.dev/features/4217>
*TAG review*
https://github.com/w3ctag/design-reviews/issues/1251
*TAG review status*
Pending
*Goals for experimentation*
Validate that JS Self-Profiling markers provide actionable
performance signal for large real-world web applications when
collected from real users, and confirm the marker data flows
correctly end-to-end through production RUM pipelines.
Specifically, we want to learn: - Whether per-sample markers let
developers attribute slow field traces to browser activity outside
their JavaScript (rendering, style/layout, GC) and act on it. -
Whether the sampled marker data is accurate and stable enough at
production scale (real devices, real workloads) to drive
optimization decisions. - Whether the API shape
(ProfilerSample.marker) is ergonomic for existing self-profiling
consumers to adopt with minimal code change. Interested partners
include Excel Online, which will consume the trial to enrich its
existing JS self-profiling telemetry, and Datadog, which collects
JS self-profiling data as part of its RUM product.
*Origin Trial documentation link*
https://github.com/WICG/js-self-profiling/blob/main/markers.md
*Risks*
*Interoperability and Compatibility*
/No information provided/
/Gecko/: No signal
/WebKit/: No signal
/Web developers/: No signals
/Other signals/:
*Ergonomics*
Low. Markers add a single optional marker field to the existing
ProfilerSample, no new API surface or call pattern. The main
ergonomic subtlety is graduated disclosure: style/layout are
available everywhere, while script, gc, and paint appear only in
cross-origin-isolated documents. The same profiling code therefore
yields different marker sets depending on the document's isolation
state, which could surprise developers who expect the full set.
This degrades gracefully, a missing/absent marker is simply
interpreted as idle/unknown, so it does not break existing traces,
and analysis tooling can treat markers as best-effort annotations
*Activation*
Low to moderate. The feature is a small additive extension to the
already-shipped JS Self-Profiling API, so adoption cost is minimal
for developers already using it. The main activation barrier is
that the restricted markers (script/gc/paint) require cross-origin
isolation (COOP: same-origin + COEP: require-corp), which some
large applications cannot easily deploy. This is mitigated by
exposing the safe style/layout markers in non-isolated contexts,
so developers get useful signal even without COI, and by the
Origin Trial allowing partners to evaluate the feature on stable
before committing to isolation.
*Security*
Moderate, and mitigated by design. Some browser work is shared
across documents in a process, so exposing it could leak
cross-origin timing: gc is not attributable to a single origin,
and paint could act as a proxy for the render time of cross-origin
opaque resources that fail a TAO check. To mitigate, script, gc,
and paint markers are gated behind cross-origin isolation, in a
COI process the document can already observe process-wide signals
(Spectre threat model, performance.measureMemory, WeakRef), so
these markers add no new leverage. style and layout are exposed
without isolation because their timing is already observable
through existing DOM/CSSOM APIs (getBoundingClientRect,
getComputedStyle), consistent with Long Animation Frames. Markers
attributable to a document are only attached when that document is
same-origin with the profiler. Security discussion is tracked in
https://github.com/WICG/js-self-profiling/issues/61 (Chrome
security reviewed the COI-gated design in 2021, the non-isolated
style/layout exposure is under active review).
*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/
*Ongoing technical constraints*
The restricted markers (script, gc, paint) remain permanently
gated behind cross-origin isolation; only style and layout are
exposed in non-isolated documents. Markers are best-effort,
sampled annotations (≈10 ms interval), attached only when the
responsible document is same-origin with the profiler — they are
not a precise or guaranteed signal and must not be relied upon for
correctness.
*Debuggability*
/No information provided/
*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
<https://chromium.googlesource.com/chromium/src/+/main/docs/testing/web_platform_tests.md>?*
Yes
https://wpt.fyi/results/js-self-profiling/tentative/marker-vm-state.https.html
*DevTrial instructions*
https://github.com/victorhuangwq/js-profiler-markers-demo
*Flag name on about://flags*
--enable-blink-features=ExperimentalJSProfilerMarkers
*Finch feature name*
JSSelfProfilingMarkers
*Requires code in //chrome?*
False
*Tracking bug*
https://issues.chromium.org/issues/40800459
*Estimated milestones*
Origin trial desktop first 152
*Link to entry on the Chrome Platform Status*
https://chromestatus.com/feature/5201297767792640?gate=5361868559482880
This intent message was generated by Chrome Platform Status
<https://chromestatus.com>.
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