Angular CLI v20.1 PostCSS Integration Changes Break Third-Party Build Systems - Need Migration Guidance

[JMGomes]

Command

other

Is this a regression?

• Yes, this behavior used to work in the previous version

The previous version in which this bug was not present was

v20.0.6

Description

Angular CLI v21's PostCSS integration changes (commit 49a0973) have broken Tailwind CSS processing in third-party build systems like Bazel + Aspect Rules JS. The removal of the unified Tailwind detection system and introduction of PostCSS config discovery created an ecosystem compatibility gap.

This isn't a request to fix third-party tools, but rather:

• Documentation request: How should build system integrators adapt to v21 changes?
• Ecosystem impact awareness: Significant breaking change affecting Angular + Bazel projects
• Migration guidance: What's the recommended integration pattern for the new PostCSS system?

Minimal Reproduction

1. setup a new angular project with tailwind
2. add tailwind directive
3. configure bazel
4. compare styles.css from ng build --configuration production vs bazel build //:web-app-build

Exception or Error

No runtime errors, but silently broken functionality. 
tailwind are not processed

Your Environment

Angular CLI: v21.0.0+
Node.js: v22.12.0
Bazel: Latest with Aspect Rules JS
Tailwind CSS: v3.3.5
PostCSS: v8.5.6
OS: Linux 6.8.0-64-generic

Anything else relevant?

Root Cause Analysis:
The v21 changes introduced two new systems that third-party build tools don't implement - 49a0973

1. PostCSS Config Discovery - Searches for postcss.config.json, .postcssrc.json
2. Modified Automatic Detection - New fallback logic when no config found

Evidence from Investigation:
✅ File discovery works: Bazel finds tailwind.config.js
✅ Dependencies available: All PostCSS/Tailwind packages present
✅ Same execution environment: Verified via sandbox debugging
❌ Processing pipeline broken: Third-party tools missing new integration logic

Impact on Ecosystem:

• Affects all Angular projects using Bazel builds with Tailwind
• Creates build system inconsistency (same code, different output)
• No migration documentation exists for build system integrators
• Forces workaround of using only ng build

Requests for Angular CLI Team:
📚 Documentation: How should third-party build systems implement the new PostCSS discovery?
🛠️ Migration Guide: What's the recommended integration pattern for build tools?

[clydin]

Angular version v21 does not yet exist and is tentatively scheduled to be released the week of 2025-11-17. Assuming this is rather regarding v20.1, there is currently fallback logic present for tailwind if no postcss configuration is present. This change in code mention above also only applies to the Webpack-based browser builder. The application build system that is the default for all new projects and which already operates in this manner, has had no behavioral changes in this area between the versions mentioned.
Please also note that Talwind v4 has breaking changes that require the use of the postcss configuration method as indicated by the official Tailwind documentation for Angular: https://tailwindcss.com/docs/installation/framework-guides/angular

[jkrems]

Trying to parse the issue description: It sounds like there's a bazel rule that needs to be updated to support tailwind v4 with the browser builder..? The upstream issue on their end appears to be aspect-build/rules_js#2290. But to me it seems like the diagnosis of the issue is way off. I suspect what happened here is "user created a new file that's required for the build but didn't add it as a dep of the bazel target running the build". Which I'm not sure is really a bug on anybody's side, it's just how making changes to bazel-based projects works.

If you have a repro where the CLI upgrade itself (without also upgrading tailwind and adding new files) broke tailwind, that would definitely be an issue on the end of the Angular CLI and/or the build rule. But bazel doing different things if you pass it different files is WAI for bazel afaict.