Change PostCSS version strategy

.github/bin/format-package-json.mjs

@@ -1,6 +1,29 @@
 import { promises as fsp, constants } from 'fs';
 import path from 'path';
 
+async function postcssPeerDependencyVersion() {
+	// "postcss-tape" is our reference for PostCSS versions.
+	// This package is our test suite.
+	// If CI passes it means the plugin is compatible with the lower version in "postcss-tape".
+	const packageJSONInfoForPostCSS_Tape = JSON.parse(await fsp.readFile('../../packages/postcss-tape/package.json', 'utf8'));
+
+	// "postcss-tape" package.json:
+	//
+	// "dependencies": {
+	//   "postcss": "~8.4",
+	//   "postcss-8.2": "npm:postcss@~8.2"
+	// }
+	//
+	const lowerPostCSS_VersionKey = Object.keys(packageJSONInfoForPostCSS_Tape.dependencies).find((x) => {
+		return x.startsWith('postcss-') && packageJSONInfoForPostCSS_Tape.dependencies[x].includes('npm:postcss@~');
+	});
+
+	const lowerPostCSS_Version = packageJSONInfoForPostCSS_Tape.dependencies[lowerPostCSS_VersionKey];
+
+	// "npm:postcss@~8.2" -> "8.2"
+	return lowerPostCSS_Version.split('~')[1];
+}
+
 const packageJSONInfo = JSON.parse(await fsp.readFile('./package.json', 'utf8'));
 const packageJSONInfoCopy = JSON.stringify(packageJSONInfo, null, '\t');
 const formatted = {};
@@ -98,6 +121,10 @@ const formatted = {};
 			formatted.peerDependencies[dependencyKeys[i]] = packageJSONInfo.peerDependencies[dependencyKeys[i]];
 		}
 		delete packageJSONInfo.peerDependencies;
+
+		if (formatted.peerDependencies['postcss']) {
+			formatted.peerDependencies['postcss'] = '^' + (await postcssPeerDependencyVersion());
+		}
 	}
 
 	if (Object.keys(packageJSONInfo.devDependencies ?? {}).length) {

UPDATING_POSTCSS.md

@@ -2,13 +2,40 @@
 
 see : `packages/postcss-tape/package.json`
 
-Set the `postcss` version to match the peer dependency of all plugins.<br>
-Set the `postcss-oldest-supported` depending on community needs. This can be the same as the `postcss` version.
+```js
+"dependencies": {
+	// most recent version
+	"postcss": "~8.4",
+	// oldest supported
+	"postcss-8.2": "npm:postcss@~8.2"
+},
+```
 
+## updating
+
+- only update `postcss-tape`
+- run `npm run lint` from the root
+
+This will update the peer dependency version for PostCSS in all packages and plugins.
+
+## most recent version
+
+- kept up to date
+- only used in CI to catch regressions
+
+## oldest supported
+
+- used as the peer dependency version in all plugins and packages
+- updated based on community needs
+- can be the same as the **most recent version** version
+
+## semver flags
 
 - Plugins use `^MAJOR.MINOR`.
 - PostCSS Tape uses `~MAJOR.MINOR`.
 
+## notes
+
 Avoid increasing these versions when possible.
 
 Major version updates are also always a breaking change.

experimental/css-has-pseudo/package.json

@@ -40,7 +40,7 @@
 		"postcss-selector-parser": "^6.0.10"
 	},
 	"peerDependencies": {
-		"postcss": "^8.3"
+		"postcss": "^8.2"
 	},
 	"devDependencies": {
 		"@mrhenry/core-web": "^0.7.2",