310264 2026-03-19 01:35:18 -0700 LCP in shadow DOM is not reported 2026-04-06 11:38:23 -0700 1 1 1 Unclassified WebKit Layout and Rendering WebKit Nightly Build Unspecified Unspecified REOPENED InRadar P2 Normal --- 311575 1 smrtnyk simon.fraser barry bfulgham commit-queue erik.witt karlcow nicole simon.fraser webkit-bug-importer zalan oldest_to_newest 2191646 0 478720 smrtnyk 2026-03-19 01:35:18 -0700 Created attachment 478720 you will need to install playwright test for this Both chrome and firefox report LCP coming from shadow dom but webkit doesn't. Given a scenario if image is loaded from shadow dom and is the best LCP candidate, chrome and firefox will report it as an lcp where webkit will ignore it and take only candidates from regular DOM. I am not really sure if webkit is more behaving to spec or not. It could be that firefox aligned to Chrome and webkit decided to follow the spec more closely. Relevant links: • LCP spec: https://w3c.github.io/largest-contentful-paint/ (section 4.2) • Paint Timing spec: https://w3c.github.io/paint-timing/ (section 5.4.1 "Exposed for paint timing") The LCP spec section 4.2 step 5.2 says If imageElement is not exposed for paint timing, given document, continue. in Paint timing spec seciton 5.4.1 step 4 If element’s root is not equal to document, return false. I am not sure how to interpret it. I have attached a test script you can run via node It uses @playwright/test in order to run all 3 browsers. 2192948 1 simon.fraser 2026-03-23 17:40:28 -0700 I thought there was a web platform test for this that we passed. 2192949 2 webkit-bug-importer 2026-03-23 17:40:48 -0700 <rdar://problem/173197678> 2196739 3 simon.fraser 2026-04-03 17:16:19 -0700 Pull request: https://github.com/WebKit/WebKit/pull/62008 2196870 4 ews-feeder 2026-04-04 08:35:43 -0700 Committed 310575@main (0b5c61add5d5): <https://commits.webkit.org/310575@main> Reviewed commits have been landed. Closing PR #62008 and removing active labels. 2196970 5 ews-feeder 2026-04-04 21:13:00 -0700 Committed 305413.615@safari-7624-branch (c1f3664d9ebb): <https://commits.webkit.org/305413.615@safari-7624-branch> Reviewed commits have been landed. Closing PR #4896 and removing active labels. 2197295 6 commit-queue 2026-04-06 11:27:19 -0700 Re-opened since this is blocked by bug 311575 2197297 7 simon.fraser 2026-04-06 11:29:16 -0700 It's not clear from specs about whether this is correct, so reverted. 2197301 8 simon.fraser 2026-04-06 11:38:23 -0700 https://github.com/w3c/largest-contentful-paint/issues/152 478720 2026-03-19 01:35:18 -0700 2026-03-19 01:35:18 -0700 you will need to install playwright test for this webkit-lcp-shadow-dom-bug.mjs text/javascript 4102 smrtnyk LyoqDQogKiBXZWJLaXQgZG9lcyBub3QgcmVwb3J0IHNoYWRvdyBET00gY29udGVudCBhcyBMQ1Ag KExhcmdlc3QgQ29udGVudGZ1bCBQYWludCkgY2FuZGlkYXRlcy4NCiAqDQogKiBFeHBlY3RlZCAo Q2hyb21pdW0gJiBGaXJlZm94KToNCiAqICAgLSBMQ1AgZW50cnkgZm9yIHRoZSBzaGFkb3cgRE9N IGltYWdlOiBzaXplPTQ4MDAwMCwgZWxlbWVudD1udWxsLCB1cmw9PGltYWdlIHVybD4NCiAqDQog KiBBY3R1YWwgKFdlYktpdCk6DQogKiAgIC0gU2hhZG93IERPTSBpbWFnZSBpcyBuZXZlciByZXBv cnRlZCBhcyBhbiBMQ1AgY2FuZGlkYXRlLg0KICogICAtIExDUCBlbnRyeSBpcyBmb3IgdGhlIHNt 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