Add stylelint CSS linting

.stylelintignore

@@ -0,0 +1,27 @@
+# Ignore WordPress plugin files.
+src/wp-content/plugins/**/*
+
+# Ignore WordPress theme files.
+src/wp-content/themes/**/*
+
+# Ignore WordPress block files.
+src/wp-includes/blocks/**/*
+
+# Ignore admin color schemes auto generated files.
+src/wp-admin/css/colors/*/*.css
+
+# Ignore WordPress included dist files.
+src/wp-includes/css/dist/**/*
+
+# Ignore files copied to wp-includes js.
+src/wp-includes/js/**/*
+
+# Ignore WordPress JS vendor files.
+src/js/_enqueues/vendor/**/*
+
+# Ignore minified CSS files.
+**/*.min.css
+
+# Ignore RTL CSS files.
+**/*-rtl.css
+**/*-rtl.min.css

.stylelintrc.js

@@ -0,0 +1,106 @@
+/** @type {import('stylelint').Config} */
+module.exports = {
+	extends: '@wordpress/stylelint-config/scss-stylistic',
+	plugins: [
+		'stylelint-plugin-logical-css',
+		'@wordpress/theme/stylelint-plugins/no-token-fallback-values',
+	],
+	reportNeedlessDisables: true,
+	rules: {
+		'at-rule-empty-line-before': [
+			'always',
+			{
+				except: [ 'blockless-after-blockless' ],
+				ignore: [ 'first-nested', 'after-comment' ],
+			},
+		],
+		'at-rule-no-unknown': null,
+		'comment-empty-line-before': null,
+		'declaration-property-unit-allowed-list': [
+			{
+				'line-height': [],
+			},
+		],
+		'declaration-property-value-allowed-list': [
+			{
+				'flex-direction': '/^(?!(row|column)-reverse).*$/',
+			},
+			{
+				message: ( property, value ) =>
+					`Avoid "${ value }" value for the "${ property }" property. For accessibility reasons, visual, reading, and DOM order must match. Only use the reverse values when they do not affect reading order, meaning, and interaction.`,
+			},
+		],
+		'font-family-no-missing-generic-family-keyword': [
+			true,
+			{
+				ignoreFontFamilies: [ 'dashicons' ],
+			},
+		],
+		'font-weight-notation': 'numeric',
+		'function-disallowed-list': [ 'rgb' ],
+		'no-descending-specificity': null,
+		'no-invalid-position-at-import-rule': null,
+		'property-disallowed-list': [
+			[ 'order' ],
+			{
+				message:
+					'Avoid the order property. For accessibility reasons, visual, reading, and DOM order must match. Only use the order property when it does not affect reading order, meaning, and interaction.',
+			},
+		],
+		'rule-empty-line-before': [
+			'always',
+			{
+				except: [ 'first-nested' ],
+				ignore: [ 'after-comment' ],
+			},
+		],
+		'selector-class-pattern': [
+			'^([a-z][a-z0-9]*)(-[a-z0-9]+)*$',
+			{
+				message:
+					'Class selector should use lowercase and separate words with hyphens',
+			},
+		],
+		'selector-id-pattern': [
+			'^([a-z][a-z0-9]*)(-[a-z0-9]+)*$',
+			{
+				message:
+					'ID selector should use lowercase and separate words with hyphens',
+			},
+		],
+		'selector-no-qualifying-type': true,
+		'value-keyword-case': [
+			'lower',
+			{
+				ignoreKeywords: [
+					'currentColor',
+					'optimizeLegibility',
+				],
+			},
+		],
+		'scss/at-else-empty-line-before': null,
+		'scss/at-extend-no-missing-placeholder': null,
+		'scss/at-if-closing-brace-newline-after': null,
+		'scss/at-if-closing-brace-space-after': null,
+		'scss/comment-no-empty': null,
+		'scss/load-partial-extension': null,
+		'scss/no-global-function-names': null,
+		'scss/operator-no-newline-after': null,
+		'scss/operator-no-unspaced': null,
+		'scss/selector-no-redundant-nesting-selector': null,
+		'@stylistic/max-empty-lines': 1,
+		'@stylistic/max-line-length': null,
+		'@stylistic/selector-descendant-combinator-no-non-space': true,
+		// Keep these lines. Don't change their order otherwise indentation will break.
+		// It's not about a literal 'execution order', but rather a compatibility issue.
+		'@stylistic/block-closing-brace-newline-before': 'always',
+		'@stylistic/block-opening-brace-newline-after': 'always',
+		'@stylistic/declaration-block-semicolon-newline-after': 'always',
+		'@stylistic/selector-list-comma-space-after': 'always-single-line',
+		'@stylistic/selector-list-comma-newline-after': 'always',
+		'@stylistic/declaration-colon-newline-after': 'always-multi-line',
+		'@stylistic/indentation': 'tab',
+		// End keep these lines.
+	},
+	reportDescriptionlessDisables: true,
+};

Gruntfile.js

@@ -1823,8 +1823,21 @@ module.exports = function(grunt) {
 		'qunit:compiled'
 	] );
 
+	grunt.registerTask( 'lint:css', 'Runs Stylelint on core CSS.', function() {
+		var done = this.async();
+
+		grunt.util.spawn( {
+			cmd: 'node',
+			args: [ 'tools/stylelint/lint-css.js' ],
+			opts: { stdio: 'inherit' }
+		}, function( error ) {
+			done( ! error );
+		} );
+	} );
+
 	grunt.registerTask( 'precommit:css', [
-		'postcss:core'
+		'postcss:core',
+		'lint:css',
 	] );
 
 	grunt.registerTask( 'precommit:php', [
@@ -1921,7 +1934,7 @@ module.exports = function(grunt) {
 				}
 
 				if ( code === 0 ) {
-					if ( [ 'package.json', 'Gruntfile.js', 'composer.json' ].some( testPath ) ) {
+					if ( [ 'package.json', 'Gruntfile.js', 'composer.json', '.stylelintrc.js', '.stylelintignore' ].some( testPath ) ) {
 						grunt.log.writeln( 'Configuration files modified. Running `prerelease`.' );
 						taskList.push( 'prerelease' );
 					} else {