Build failure: CSS modules with data URLs

This issue was created by an agent analysing CI failures from the Next.js Deploy Suite (vinext main vs Next.js v16.2.6, 2026-05-20).

Problem

CSS modules that use data URLs (e.g., composes: classname from "data:text/css,..." or inline CSS data URIs) fail the Vite build entirely.

All test assertions fail with Custom deploy script failed: undefined (1), indicating a build-time failure before any tests ran.

Estimated Impact

~4 test failures (build failure blocks the entire suite).

Affected Test Suites

• test/e2e/app-dir/css-modules-data-urls/css-modules-data-urls.test.ts (4 failures)

Recommendation

1. Reproduce first in vinext's own test suite. Add a test with a CSS module that uses composes from a data URL and verify the build fails.

2. Investigate the Vite CSS module pipeline. Data URLs in CSS modules may not be supported by Vite's CSS processing or by LightningCSS. Check if there's a Vite plugin or configuration that enables data URL support in CSS modules.

3. Note: This may be related to the Rolldown CSS data URL bundling removal (tracked separately in #1343 for global CSS). The CSS modules case may need a different fix.