Merge commit '9b0050e9aabe4be65c78ccf292a348f309d50ccd' as 'docs'

```
git subtree add --prefix=docs/ https://github.com/gohugoio/hugoDocs.git master --squash
```

Closes #11925

docs/content/en/functions/resources/PostProcess.md

@@ -0,0 +1,160 @@
+---
+title: resources.PostProcess
+description: Processes the given resource after the build.
+categories: []
+keywords: []
+action:
+  aliases: []
+  related:
+    - functions/resources/Fingerprint
+    - functions/resources/Minify
+    - functions/resources/PostCSS
+    - functions/resources/ToCSS
+  returnType: postpub.PostPublishedResource
+  signatures: [resources.PostProcess RESOURCE]
+toc: true
+---
+
+```go-html-template
+{{ with resources.Get "css/main.css" }}
+  {{ if hugo.IsDevelopment }}
+    <link rel="stylesheet" href="{{ .RelPermalink }}">
+  {{ else }}
+    {{ with . | postCSS | minify | fingerprint | resources.PostProcess }}
+      <link rel="stylesheet" href="{{ .RelPermalink }}" integrity="{{ .Data.Integrity }}" crossorigin="anonymous">
+    {{ end }}
+  {{ end }}
+{{ end }}
+```
+
+Marking a resource with `resources.PostProcess` postpones transformations until the build has finished.
+
+Call `resources.PostProcess` when one or more of the steps in the transformation chain depends on the result of the build.
+
+A prime use case for this is purging unused CSS rules using the [PurgeCSS] plugin for the PostCSS Node.js package.
+
+## CSS Purging
+
+{{% note %}}
+There are several ways to set up CSS purging with PostCSS in Hugo. If you have a simple project, you should consider going the simpler route and drop the use of `resources.PostProcess` and just extract keywords from the templates. See the [Tailwind documentation](https://tailwindcss.com/docs/controlling-file-size/#app) for examples.
+{{% /note %}}
+
+Step 1
+: Install [Node.js].
+
+Step 2
+: Install the required Node.js packages in the root of your project:
+
+```sh
+npm i -D postcss postcss-cli autoprefixer @fullhuman/postcss-purgecss
+```
+
+Step 3
+: Create a PostCSS configuration file in the root of your project. You must name this file `postcss.config.js` or another [supported file name]. For example:
+
+```js
+const autoprefixer = require('autoprefixer');
+const purgecss = require('@fullhuman/postcss-purgecss')({
+  content: ['./hugo_stats.json'],
+  defaultExtractor: content => {
+    const els = JSON.parse(content).htmlElements;
+    return [
+      ...(els.tags || []),
+      ...(els.classes || []),
+      ...(els.ids || []),
+    ];
+  },
+  // https://purgecss.com/safelisting.html
+  safelist: []
+});
+
+module.exports = {
+  plugins: [
+    autoprefixer,
+    process.env.HUGO_ENVIRONMENT !== 'development' ? purgecss : null
+  ]
+};
+```
+
+{{% note %}}
+{{% include "functions/resources/_common/postcss-windows-warning.md" %}}
+{{% /note %}}
+
+Step 4
+: Enable creation of the `hugo_stats.json` file when building the site. If you are only using this for the production build, consider placing it below [config/production].
+
+{{< code-toggle file=hugo >}}
+[build.buildStats]
+enable = true
+{{< /code-toggle >}}
+
+See the [configure build] documentation for details and options.
+
+Step 5
+: Place your CSS file within the `assets/css` directory.
+
+Step 6
+: If the current environment is not `development`, process the resource with PostCSS:
+
+```go-html-template
+{{ with resources.Get "css/main.css" }}
+  {{ if hugo.IsDevelopment }}
+    <link rel="stylesheet" href="{{ .RelPermalink }}">
+  {{ else }}
+    {{ with . | postCSS | minify | fingerprint | resources.PostProcess }}
+      <link rel="stylesheet" href="{{ .RelPermalink }}" integrity="{{ .Data.Integrity }}" crossorigin="anonymous">
+    {{ end }}
+  {{ end }}
+{{ end }}
+```
+
+## Environment variables
+
+Hugo passes these environment variables to PostCSS, which allows you to do something like:
+
+```js
+process.env.HUGO_ENVIRONMENT === 'production' ? [autoprefixer] : []
+```
+
+PWD
+: The absolute path to the project working directory.
+
+HUGO_ENVIRONMENT
+: The current Hugo environment, set with the `--environment` command line flag.
+Default is `production` for `hugo` and `development` for `hugo server`.
+
+HUGO_PUBLISHDIR
+: The absolute path to the publish directory (the `public` directory). Note that the value will always point to a directory on disk even when running `hugo server` in memory mode. If you write to this folder from PostCSS when running the server, you could run the server with one of these flags:
+
+```sh
+hugo server --renderToDisk
+hugo server --renderStaticToDisk
+```
+
+Also, Hugo will add environment variables for all files mounted below `assets/_jsconfig`. A default mount will be set up with files in the project root matching this regexp: `(babel|postcss|tailwind)\.config\.js`.
+
+These will get environment variables named on the form `HUGO_FILE_:filename:` where `:filename:` is all upper case with periods replaced with underscore. This allows you to do something like:
+
+```js
+let tailwindConfig = process.env.HUGO_FILE_TAILWIND_CONFIG_JS || './tailwind.config.js';
+```
+
+## Limitations
+
+Do not use `resources.PostProcess` when running Hugo's built-in development server. The examples above specifically prevent this by verifying that the current environment is not "development".
+
+The `resources.PostProcess` function only works within templates that produce HTML files.
+
+You cannot manipulate the values returned from the resource’s methods. For example, the `strings.ToUpper` function in this example will not work as expected:
+
+```go-html-template
+{{ $css := resources.Get "css/main.css" }}
+{{ $css = $css | resources.PostCSS | minify | fingerprint | resources.PostProcess }}
+{{ $css.RelPermalink | strings.ToUpper }}
+```
+
+[node.js]: https://nodejs.org/en/download
+[supported file name]: https://github.com/postcss/postcss-load-config#usage
+[config/production]: /getting-started/configuration/#configuration-directory
+[configure build]: /getting-started/configuration/#configure-build
+[purgecss]: https://github.com/FullHuman/purgecss#readme