Add keyframe-block-no-duplicate-selectors

docs/user-guide/rules/list.md

@@ -50,6 +50,10 @@ Within each cateogory, the rules are grouped by the [_thing_](http://apps.workfl
 
 - [`keyframe-declaration-no-important`](../../../lib/rules/keyframe-declaration-no-important/README.md): Disallow `!important` within keyframe declarations.
 
+### Keyframe blocks
+
+- [`keyframe-block-no-duplicate-selectors`](../../../lib/rules/keyframe-block-no-duplicate-selectors/README.md): Disallow duplicate selectors within keyframe blocks.
+
 ### Declaration block
 
 - [`declaration-block-no-duplicate-custom-properties`](../../../lib/rules/declaration-block-no-duplicate-custom-properties/README.md): Disallow duplicate custom properties within declaration blocks.

lib/rules/index.js

@@ -119,6 +119,7 @@ const rules = {
 	'function-url-scheme-disallowed-list': importLazy('./function-url-scheme-disallowed-list'),
 	'function-whitespace-after': importLazy('./function-whitespace-after'),
 	'hue-degree-notation': importLazy('./hue-degree-notation'),
+	'keyframe-block-no-duplicate-selectors': importLazy('./keyframe-block-no-duplicate-selectors'),
 	'keyframe-declaration-no-important': importLazy('./keyframe-declaration-no-important'),
 	'keyframes-name-pattern': importLazy('./keyframes-name-pattern'),
 	'length-zero-no-unit': importLazy('./length-zero-no-unit'),

lib/rules/keyframe-block-no-duplicate-selectors/README.md

@@ -0,0 +1,40 @@
+# keyframe-block-no-duplicate-selectors
+
+Disallow duplicate selectors within keyframe blocks.
+
+<!-- prettier-ignore -->
+```css
+@keyframes foo {
+  0% {}
+  0% {}  /* This duplicated selector */
+  100% {}
+}
+```
+
+## Options
+
+### `true`
+
+The following patterns are considered problems:
+
+<!-- prettier-ignore -->
+```css
+@keyframes foo { 0% {} 0% {} 100% {} }
+```
+
+<!-- prettier-ignore -->
+```css
+@keyframes foo { 0% {} 0%, 100% {} }
+```
+
+The following patterns are _not_ considered problems:
+
+<!-- prettier-ignore -->
+```css
+@keyframes foo { 0% {} 100% {} }
+```
+
+<!-- prettier-ignore -->
+```css
+@keyframes foo { from {} to {} }
+```

lib/rules/keyframe-block-no-duplicate-selectors/__tests__/index.js

@@ -0,0 +1,40 @@
+'use strict';
+
+const { messages, ruleName } = require('..');
+
+testRule({
+	ruleName,
+	config: [true],
+
+	accept: [
+		{
+			code: '@keyframes foo { 0% {} 100% {} }',
+		},
+		{
+			code: '@keyframes foo { from {} to {} }',
+		},
+	],
+
+	reject: [
+		{
+			code: '@keyframes foo { from {} to {} to {} }',
+			message: messages.rejected('to'),
+		},
+		{
+			code: '@keyframes foo { 0% {} 0% {} 100% {} }',
+			message: messages.rejected('0%'),
+		},
+		{
+			code: '@-webkit-keyframes foo { 0% {} 0% {} 100% {} }',
+			message: messages.rejected('0%'),
+		},
+		{
+			code: '@-moz-keyframes foo { 0% {} 0% {} 100% {} }',
+			message: messages.rejected('0%'),
+		},
+		{
+			code: '@keyframes foo { 0% {} 0%, 100% {} }',
+			message: messages.rejected('0%'),
+		},
+	],
+});

lib/rules/keyframe-block-no-duplicate-selectors/index.js

@@ -0,0 +1,56 @@
+'use strict';
+
+const report = require('../../utils/report');
+const ruleMessages = require('../../utils/ruleMessages');
+const validateOptions = require('../../utils/validateOptions');
+
+const ruleName = 'keyframe-block-no-duplicate-selectors';
+
+const messages = ruleMessages(ruleName, {
+	rejected: (selector) => `Unexpected duplicate "${selector}"`,
+});
+
+const meta = {
+	url: 'https://stylelint.io/user-guide/rules/list/keyframe-block-no-duplicate-selectors',
+};
+
+/** @type {import('stylelint').Rule} */
+const rule = (primary) => {
+	return (root, result) => {
+		const validOptions = validateOptions(result, ruleName, { actual: primary });
+
+		if (!validOptions) {
+			return;
+		}
+
+		root.walkAtRules(/^(-(moz|webkit)-)?keyframes$/i, (atRuleKeyframes) => {
+			const selectors = new Set();
+
+			atRuleKeyframes.walkRules((keyframeRule) => {
+				const ruleSelectors = keyframeRule.selectors;
+
+				ruleSelectors.forEach((selector) => {
+					const isDuplicate = selectors.has(selector);
+
+					if (isDuplicate) {
+						report({
+							message: messages.rejected(selector),
+							node: keyframeRule,
+							result,
+							ruleName,
+						});
+
+						return;
+					}
+
+					selectors.add(selector);
+				});
+			});
+		});
+	};
+};
+
+rule.ruleName = ruleName;
+rule.messages = messages;
+rule.meta = meta;
+module.exports = rule;