cloudflare/vinext

Build: `experimental.lightningCssFeatures` exclude list not honored

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#1,498 opened on May 22, 2026

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Description

This issue was created by an agent analysing CI failures from the Next.js Deploy Suite (vinext main vs Next.js v16.2.6, 2026-05-22).

Problem

Setting lightningCssFeatures to exclude light-dark() should preserve the CSS function untransformed. vinext still transforms it into var(--lightningcss-light, ...) polyfill output, ignoring the exclude config.

Excluded `light-dark()` still transformed by LightningCSS

Estimated Impact

~1 test failures across the deploy suite.

Affected Test Suites

  • test/e2e/app-dir/experimental-lightningcss-features/experimental-lightningcss-features.test.ts

Recommendation

  1. Reproduce first in vinext's own test suite. Configure experimental: { lightningCssFeatures: { exclude: ['light-dark'] } } and assert the function survives in the output.

  2. Plumb the exclude list into LightningCSS. Read lightningCssFeatures.exclude and pass it as the corresponding exclude bitmask/options when calling LightningCSS.

Contributor guide

Research direction
Reproduce the test failure locally, then trace how `lightningCssFeatures.exclude` is consumed in the LightningCSS integration code to identify where the exclusion logic is being bypassed.
Tech stack
typescriptnodejscss
Domain
build systemtooling
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
GitNode.js
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
75