Have you read the Contributing Guidelines on issues?

• I have read the Contributing Guidelines on issues.

Prerequisites

• I'm using the latest version of Docusaurus.
• I have tried the npm run clear or yarn clear command.
• I have tried rm -rf node_modules yarn.lock package-lock.json and re-installing packages.
• I have tried creating a repro with https://new.docusaurus.io.
• I have read the console error message carefully (if applicable).

Description

Docusaurus 3.4.0+ tries to minimize CSS files from static directory, while Docusaurus 3.3.2 and earlier just kept them intact. While doing that CSS minimization, Docusaurus 3.4.0+ forgets to initialize PostCSS correctly, ignoring custom plugins like they do not exist. This leads to numerous issues in projects that use CSS 3 syntax in static CSS files, because they cannot be built without producing warnings and errors.

This is a regressive behavior. The same projects worked in Docusaurus 3.3.2 and earlier. An alternative solution would be to instruct Docusaurus 3.4.0+ to exclude CSS files from static folder minimization.

Reproducible demo

https://github.com/hrumhurum/docusaurus-postcss-issue-repro

Steps to reproduce

1. Enter the directory of a corresponding sample:

   cd 3.3.2 or
   cd 3.4.0

2. Install the missing dependencies. This creates node_modules directory which is not
   included in this repository:

   npm install

3. Build the project

   npm run build

Expected behavior

• Build finishes successfully

Actual behavior

• [WARNING] {"file":"api/my-api.css","message":"api/my-api.css from Css Minimizer plugin\nUnexpected '}' at api/my-api.css:1:85.","compilerPath":"client"}
• [WARNING] {"file":"api/my-api.css","message":"api/my-api.css from Css Minimizer plugin\nInvalid character(s) '}' at api/my-api.css:1:85. Ignoring.","compilerPath":"client"}
• [WARNING] {"file":"api/my-api.css","message":"api/my-api.css from Css Minimizer plugin\nInvalid property name '& .my-api-sub-area{background' at api/my-api.css:1:40. Ignoring.","compilerPath":"client"}

Your environment

• Docusaurus 3.4.0
• Windows 11 Version 21H2 (OS Build 22000.2538)
• x64

Self-service

• I'd be willing to fix this bug myself.