[csswg-drafts] [css-2025][css-transitions-2] Add `transition-behavior`, `@starting-style`, and `CSSStartingStyleRule` to Safe to Release exceptions (#12778)

SebastianZ has just created a new issue for https://github.com/w3c/csswg-drafts:

== [css-2025][css-transitions-2] Add `transition-behavior`, `@starting-style`, and `CSSStartingStyleRule` to Safe to Release exceptions ==
Splitting the discussion from #10687 for adding the [`transition-behavior`](https://drafts.csswg.org/css-transitions-2/#transition-behavior-property) property, [`@starting-style` rule and `CSSStartingStyleRule` interface](https://drafts.csswg.org/css-transitions-2/#defining-before-change-style) of [CSS Transitions 2](https://drafts.csswg.org/css-transitions-2/) to the Safe to Release pre-CR Exceptions section of the CSS Snapshot 2025.

Data to consider:

* Spec. status: WD (last published: 5 September 2023)
* Implemented in all major browsers as [part of Interop 2024](https://wpt.fyi/results/css?label=experimental&label=master&aligned&view=interop&q=label%3Ainterop-2024-starting-style%20or%20label%3Ainterop-2024-transition-behavior)
* [WPT](https://wpt.fyi/results/css/css-transitions?label=master&label=experimental&aligned&q="transition-behavior"%20or%20"starting-style"%20or%20"startingstyle"): passing tests: Chrome: 60, Edge: 60, Firefox: 59, Safari: 60; [interop](https://wpt.fyi/results/css/css-transitions?label=master&label=experimental&aligned&q="transition-behavior"%20or%20"starting-style"%20or%20"startingstyle"%20all(status%3Apass)): 59 / 60 (98.3%)
* Open issues: [on GitHub](https://github.com/w3c/csswg-drafts/issues?q=is%3Aopen%20label%3Acss-transitions-2%20("transition-behavior"%20OR%20"starting-style"%20OR%20"CSSStartingStyleRule")): 6, [in spec.](https://drafts.csswg.org/css-transitions-2/#issues-index): 0

Summary: Very high interoperability, few open issues

Sebastian

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/12778 using your GitHub account


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Received on Thursday, 11 September 2025 11:20:22 UTC