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[postcss-plugin] Add @layer and polyfill support via @csstools/postcss-cascade-layers#1072

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[postcss-plugin] Add @layer and polyfill support via @csstools/postcss-cascade-layers#1072
jeongminsang wants to merge 7 commits intofacebook:mainfrom
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@jeongminsang jeongminsang commented May 21, 2025 •
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What changed / motivation ?

This update refactors the useCSSLayers option to streamline the implementation of CSS layer polyfilling. We're replacing our custom, internally handled polyfill with the robust and well-maintained @csstools/postcss-cascade-layers library. This ensures a more compliant and robust solution for handling CSS layer specificity.

Linked PR/Issues

Fixes #1062

Additional Context

useCSSLayers Option Refactoring

The previously self-implemented polyfill logic has been removed:

// babal-plugin/src/index.js
if (!useLayers) {
	ltrRule = addSpecificityLevel(ltrRule, index);
	rtlRule = rtlRule && addSpecificityLevel(rtlRule, index);
}

The polyfill functionality is now powered by @csstools/postcss-cascade-layers, ensuring a more robust and compliant solution:

// babel-plugin/src/index.js
async function transformCollectedCSS(collectedCSS: string): Promise<string> {
  return (await postcss([cascadeLayers()]).process(collectedCSS)).css;
}

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@jeongminsang jeongminsang mentioned this pull request May 21, 2025
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@facebook-github-bot facebook-github-bot added the CLA Signed This label is managed by the Facebook bot. Authors need to sign the CLA before a PR can be reviewed. label May 21, 2025
necolas
necolas requested changes May 21, 2025
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Thanks for giving this a try.

The layers polyfill should be part of the babel-plugin processStylexRules method. The unit tests for that API should also be updated to snapshot the output from the 3 different options. The other packages like the CLI and postcss-plugin can then call into processStylexRules and pass an options object instead of the current boolean.

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jeongminsang commented Jun 2, 2025

@necolas,
I'm considering implementing the polyfill for @csstools/postcss-cascade-layers within the processStylexRules method, as per your review. However, this change would require processStylexRules to become an async function, which could lead to significant breaking changes across the codebase. Are these extensive changes what you intended?

@jeongminsang jeongminsang requested a review from necolas June 11, 2025 12:05
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@Jta26 Jta26 Awaiting requested review from Jta26

@mellyeliu mellyeliu Awaiting requested review from mellyeliu mellyeliu is a code owner

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@necolas necolas Awaiting requested review from necolas necolas is a code owner

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[babel-plugin] Better options for processStylexRules

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