The Angular application builder supports custom PostCSS configuration files. The builder will automatically detect and use specific postcss configuration files if present in either the project root directory or the workspace root.

To utilize a custom PostCSS configuration:

1. Create a JSON file named either postcss.config.json or .postcssrc.json in your project's root directory. Project-level configurations override workspace-level ones.

2. Define your plugins within this file using either an array or object format:

   Array Form:

   {
     "plugins": [
       "tailwindcss",
       ["rtlcss", { "useCalc": true }]
     ]
   }

   Object Form:

   {
     "plugins": {
       "tailwindcss": {},
       "rtlcss": { "useCalc": true }
     }
   }

Thanks @alan-agius4 ! In this comment I had read postCSS config wasn't supported but glad to be wrong about that.

Following these instructions unfortunately I was not able to get v4 working. Here is my postcss.config.json file at the root of my project:

{
  "plugins": {
    "@tailwindcss/postcss": {}
  }
}

The plugin name is from the tailwind docs. I'm not seeing any error messages, but it is not compiling tailwind either. I pushed these changes to the postcss branch of my test repo

@inorganik, custom postcss configuration is only available from Angular CLI versions 17.2 onwards.

@alan-agius4 The postcss config works great in Angular 17! Only issue now is the peer deps - is there a way to not have to use the force flag for every npm install/build?

You can add legacy-peer-deps=true to an .npmrc file located in your workspace.

We had a brief discussion within the team about this today. There's nothing too controversial here just a few different tasks we'll need to land. Tailwind v4 is still in alpha, so I don't think there's anything urgent here, but once it does release we'll want to:

• Update default configuration to use @tailwindcss/postcss.
• Update CSS file detection to look for @import 'tailwind';, since this seems to be an easier way of detecting which files use Tailwind.
• Consider directly supporting Tailwind in @angular/build:application instead of requiring PostCSS, this could give some performance improvments.
• Bump peer dep to include ^4.0.0.

Angular still uses the presence of a Tailwind config file to know whether or not we need to run Tailwind. That probably won't change even in v4, even if Tailwind itself no longer requires a config file.

This feature request is now candidate for our backlog! In the next phase, the community has 60 days to upvote. If the request receives more than 20 upvotes, we'll move it to our consideration list.

You can find more details about the feature request process in our documentation.