Intent to Implement: Largest contentful paint

[Yoav Weiss]

Contact emails yoav...@chromium.org, n...@chromium.org Explainer https://github.com/WICG/LargestContentfulPaint Design docs/spec Design doc. No spec just yet.

  TAG review https://github.com/w3ctag/design-reviews/issues/378 Summary Largest Contentful Paint (LCP) is a new page load metric, which describes page speed as the speed of delivering the largest contentful element to the screen. It is the high-level metric, where Element Timing is its low-level primitive, aiming to provide a meaningful result even for developers that won't go through the trouble of annotating their sites. Motivation Developers today don't have a reliable metric that correlates with their users' visual rendering experience. Existing metrics such as First Paint and First Contentful Paint focus on initial rendering, but don't take into account the importance of the painted content, and therefore may indicate times in which the user still does not consider the page useful. Largest Contentful Paint (LCP) aims to be a new page-load metric that better correlates with user experience than the existing page-load metrics, is easy to understand and reason about as well as reduces the chance of being gamed. Risks

Interoperability and Compatibility No particular compatibility risks. As for interoperability, we've heard skepticism on the necessity of the use-case from Safari folks, but agreement on it from other vendors. At the same time, we had disagreements on the right heuristics that will be needed here, and whether it makes sense to stop measuring the metric on user input. Firefox: No public signals Edge: No public signals Safari: Public skepticism Web developers: Mixed signals Some developers disagreed about the technical details, and argued that Element Timing as a low-level primitive should be enough. Other developers chimed in emphasizing the need for a default, even if heuristic, high-level measurement.

Will this feature be supported on all six Blink platforms (Windows, Mac, Linux, Chrome OS, Android, and Android WebView)? Yes Is this feature fully tested by web-platform-tests? Not yet, but it will be, to the extent that we don't bake UA heuristics into the tests. Link to entry on the Chrome Platform Status https://www.chromestatus.com/features/5666250908762112