As discussed with Barry Pollard offline, in all web docs we assume that pixels = CSS pixels! When it's physical pixels that should be the exception that's documented.
So after a second reflection, we finally got back to the conclusion that MDN doesn't need to be updated. Now, I am looking for my last API Owner approval, can I ping someone or approvers will show naturally? Thanks! Best, Ane Le mardi 16 décembre 2025 à 01:54:18 UTC+1, [email protected] a écrit : > LGTM2 (and thanks Barry for doing outreach to RUM providers). > On 12/15/25 4:53 a.m., Yoav Weiss (@Shopify) wrote: > > LGTM1 > > The main risk here is for tools that get their input from this data and > visualize it to users (where the data can be in either synthetic or RUM). > We mainly care about RUM data here, but even there, the direct compat risk > for actual users is minimal. In other words, this change will not create > visible breakage for users, but may create one for developers that use > tools that rely on this data. > > On Mon, Dec 15, 2025 at 10:20 AM 'Barry Pollard' via blink-dev < > [email protected]> wrote: > >> Not that I've discussed this previously with Ane and this may require >> changes to those tools using these pixels to create screenshots or >> animations of CLS. I've reached out directly to the primary tools I'm aware >> off in this space (Chrome DevTools, DebugBear, WebPageTest) as well as more >> general outreach on the likes of the Web Performance slack. Will continue >> that further now the I2S is published. > > > Thanks for the outreach! > > >> >> It's a small change to accommodate this change, is not critical (the >> worst case is the wrong area of the screenshot/animation will be >> highlighted until the change is made) and brings the Layout Instability API >> inline with other Web Platform APIs and most would expect of this. So I'm >> supportive of this change, despite this risk. >> >> Thanks, >> Barry >> >> On Friday, December 12, 2025 at 6:57:20 PM UTC [email protected] wrote: >> >>> # Contact emails >>> [email protected] >>> # Explainer >>> None (This is a small compatibility fix - the spec PR serves as the >>> explainer). The PR is a spec change, not an explainer. You can say "None" >>> or link to a separate explainer if you have one. >>> # Specification >>> https://wicg.github.io/layout-instability/ >>> # Summary >>> Change Layout Instability API attribution rectangles (`prevRect` and >>> `currentRect`) from device pixels to CSS pixels. This aligns the API >>> with other Web Platform measurement APIs (getBoundingClientRect, >>> IntersectionObserver, ResizeObserver) that use CSS pixels as the standard >>> coordinate system. >>> # Blink component >>> Blink>PerformanceAPIs >>> # Motivation >>> The current implementation uses device pixels, which: >>> - Creates inconsistencies across devices with different pixel ratios >>> - Makes it difficult for developers to correlate layout shift data with >>> other measurements >>> - Deviates from the standard coordinate system used throughout the web >>> platform >>> CSS pixels provide a consistent, device-independent unit that aligns >>> with developer expectations and simplifies working with layout >>> stability metrics alongside other DOM measurements. >>> # TAG review Not applicable - This is a small compatibility fix to an >>> existing API, not a new feature. The change aligns Layout Instability API >>> attribution rectangles with the standard CSS pixel coordinate system used >>> by other Web Platform APIs. This does not introduce new API surface or >>> capabilities. # TAG review status Not applicable # Chromium bug >>> https://issues.chromium.org/issues/399058544 >>> # Risks >>> ## Interoperability and Compatibility >>> **Interoperability risk:** Low. This is a measurement/telemetry change >>> that doesn't affect site functionality. The API is used primarily by >>> monitoring/analytics tools. >>> **Gecko:** Closed without a position ( >>> https://github.com/mozilla/standards-positions/issues/1318) - Mozilla >>> supports CSS pixels for web APIs, though they don't implement the Layout >>> Instability API. >>> **WebKit:** No signal ( >>> https://github.com/WebKit/standards-positions/issues/588) >>> **Web developers:** Positive feedback from WebPerf Slack community. The >>> change simplifies working with layout stability metrics. >>> **Compatibility risk:** Low. Primary consumers are telemetry systems >>> (RUM providers, analytics tools) that can adapt their processing. The >>> change provides more consistent, useful data. Sites using the API >>> directly will see coordinate values change but can easily adapt by >>> removing device pixel ratio conversions. >>> **Rollback plan:** Feature can be disabled via flag if unexpected >>> issues arise. >>> # Ergonomics >>> This change improves ergonomics by aligning with the CSS pixel >>> coordinate system developers already use in other APIs >>> (getBoundingClientRect, IntersectionObserver, ResizeObserver), reducing >>> confusion and simplifying debugging. >>> # Activation >>> No user activation required - this is a measurement/telemetry API. # >>> Security >>> No security concerns - this changes the unit of measurement for existing >>> data, does not expose new information. # WebView application risks >>> None. This is a measurement/telemetry change that works consistently >>> across all platforms including WebView. >>> # Debuggability >>> No changes to debuggability. The API continues to work the same way, >>> just with CSS pixel coordinates instead of device pixels. >>> # 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. Web Platform Tests have been updated to verify CSS pixel behavior >>> for attribution rectangles. >>> WPT link: >>> https://wpt.fyi/results/layout-instability/attribution-rectangles-css-pixels.html >>> # Flag name on chrome://flags >>> None (controlled by base::Feature) >>> # Finch feature name >>> None - shipping enabled by default for all users. This is a low-risk >>> compatibility fix to a measurement API with good test coverage. >>> # Non-Finch justification >>> This is a compatibility fix to align the Layout Instability API with >>> standard Web Platform coordinate systems. The change affects measurement >>> data rather than user-facing functionality, and has low risk of breakage. >>> Primary consumers are RUM/analytics tools that can adapt to the change. >>> # Requires code in //chrome? >>> No. >>> # Estimated milestones >>> Shipping on desktop: 145 >>> Shipping on Android: 145 >>> # Spec changes >>> Spec PR merged: https://github.com/WICG/layout-instability/pull/125 >>> (Clarifies that attribution rectangles use CSS pixels, consistent with >>> other Web Platform APIs) >>> # Link to entry on the Chrome Platform Status >>> https://chromestatus.com/feature/5155103518228480 >>> # Links to previous Intent discussions >>> Intent to Prototype: >>> https://groups.google.com/a/chromium.org/g/blink-dev/c/EMlUrEwx_uE/m/pxBix6wWDAAJ?utm_medium=email&utm_source=footer >>> --- >>> **This intent message was generated by Chrome Platform Status.** >>> Security and Privacy reviews have been approved via ChromeStatus gates. >>> Implementation CL: >>> https://chromium-review.googlesource.com/c/chromium/src/+/6624567 >>> --- >>> >> -- >> You received this message because you are subscribed to the Google Groups >> "blink-dev" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to [email protected]. >> To view this discussion visit >> https://groups.google.com/a/chromium.org/d/msgid/blink-dev/1a72af4d-f574-488a-913e-a98135d3afban%40chromium.org >> >> <https://groups.google.com/a/chromium.org/d/msgid/blink-dev/1a72af4d-f574-488a-913e-a98135d3afban%40chromium.org?utm_medium=email&utm_source=footer> >> . >> > -- > You received this message because you are subscribed to the Google Groups > "blink-dev" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > > To view this discussion visit > https://groups.google.com/a/chromium.org/d/msgid/blink-dev/CAOmohSKPSqnxw8PAizjj%3DT9yPLWMobxv3BOgJ7J-MEkD0_qB0w%40mail.gmail.com > > <https://groups.google.com/a/chromium.org/d/msgid/blink-dev/CAOmohSKPSqnxw8PAizjj%3DT9yPLWMobxv3BOgJ7J-MEkD0_qB0w%40mail.gmail.com?utm_medium=email&utm_source=footer> > . > > -- You received this message because you are subscribed to the Google Groups "blink-dev" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion visit https://groups.google.com/a/chromium.org/d/msgid/blink-dev/f9657176-d68f-4749-857a-fa83cda7259en%40chromium.org.
