title: PostCSS warnings when NODE_ENV=development
body: <p>I'm working on a home-grown webpack config, and one of the pieces of it is PostCSS. Here are my PostCSS versions:</p>
  <pre><code>&quot;postcss&quot;: &quot;^7.0.35&quot;,
  &quot;postcss-flexbugs-fixes&quot;: &quot;^4.2.1&quot;,
  &quot;postcss-import&quot;: &quot;^13.0.0&quot;,
  &quot;postcss-loader&quot;: &quot;^3.0.0&quot;,
  &quot;postcss-preset-env&quot;: &quot;^6.7.0&quot;,
  &quot;webpack-cli&quot;: &quot;^3.3.12&quot;,
  &quot;webpack-dev-server&quot;: &quot;^3.11.0&quot;,
  &quot;webpack&quot;: &quot;^5.4.0&quot;,
  </code></pre>
  <p>My problem is that when NODE_ENV=development, I get this message spammed in the console when running the webpack-dev-server:</p>
  <pre><code>You did not set any plugins, parser, or stringifier. Right now, PostCSS does nothing. Pick plugins for your case on https://www.postcss.parts/ and use them in postcss.config.js.
  </code></pre>
  <p>I did set plugins, here is my postcss.config.js:</p>
  <pre><code>const postcssFlexbugsFixes = require('postcss-flexbugs-fixes');
  const postcssPresetEnv = require('postcss-preset-env');
  const postcssImport = require('postcss-import');
  
  module.exports = {
      plugins: [
          postcssImport,
          postcssFlexbugsFixes,
          postcssPresetEnv({ stage: 3 })
      ]
  };
  </code></pre>
  <p>And here is how I'm using it with the postcss-loader (yes, it's in a function because of how the config is designed):</p>
  <pre><code>const postCssConfig = require('../files/postcss.config');
  const MiniCssExtractPlugin = require('mini-css-extract-plugin');
  
  const loaders = (isCssModule) =&gt; ([
      process.env.NODE_ENV === 'production' ? MiniCssExtractPlugin.loader : 'style-loader',
      {
          loader: 'css-loader',
          options: {
              importLoaders: 3,
              modules: isCssModule
          }
      },
      {
          loader: 'postcss-loader',
          options: {
              postcssOptions: postCssConfig
          }
      },
      'resolve-url-loader',
      'sass-loader'
  ]);
  </code></pre>
  <p>If I run the webpack-dev-server with everything exactly the same configuration except NODE_ENV=production, everything works. For the moment, I'm going to just disable PostCSS in development. However, this seems very strange. I'm not an expert with PostCSS, I'm wondering if it is a known issue or something I'm doing wrong.</p>