diff --git a/README.md b/README.md
index 4302671..6f755f2 100644
--- a/README.md
+++ b/README.md
@@ -130,6 +130,28 @@ nodejs --version
 npm --version
 ```
 
+### Why Node.js is needed
+
+The PHPUnit tests are written in PHP, but `prepare.php` needs Node.js and npm to prepare the WordPress checkout before the tests can run.
+
+The tests load WordPress from `src/` (`ABSPATH` in `wp-tests-config.php`). Some files that WordPress loads from there are not in version control: they were removed in [changeset 61438](https://core.trac.wordpress.org/changeset/61438), and the build creates them.
+
+`prepare.php` runs `npm install && npm run build`. For the tests, the important part is the Gutenberg step (`build:gutenberg` in the [Gruntfile](https://github.com/WordPress/wordpress-develop/blob/trunk/Gruntfile.js)):
+
+1. It uses the Gutenberg version that WordPress pins in `package.json` (`gutenberg.sha`).
+2. It downloads the pre-built Gutenberg artifact for that version from GitHub Container Registry (`ghcr.io`). The server must be able to connect to `ghcr.io`.
+3. It copies the block editor PHP files, routes, blocks, scripts, styles and `theme.json` into `src/`.
+
+If this step does not run, the tests fail while WordPress loads, for example on a missing `src/wp-includes/build/routes.php` ([#292](https://github.com/WordPress/phpunit-test-runner/issues/292)). This is why Node.js and npm are requirements, even though the tests are PHP.
+
+Versions:
+
+- Use the Node.js and npm versions in the `engines` field of `wordpress-develop/package.json`. Its `.npmrc` sets `engine-strict = true`, and `devEngines` also requires the npm version, so `npm install` stops on older versions.
+
+Extra work:
+
+- `npm run build` does more than PHPUnit needs. It also makes the production build in `build/` and minifies JavaScript and CSS. The tests do not use these files. WordPress Core runs its own PHPUnit workflow after `npm ci` and `npm run build:dev`. [#244](https://github.com/WordPress/phpunit-test-runner/issues/244) tracks ways to make this step smaller.
+
 ### PHP Composer
 
 _This is a simple example for Debian / Ubuntu._