feat: add selector support to require-baseline

docs/rules/require-baseline.md

@@ -23,6 +23,7 @@ This rule warns when it finds any of the following:
 - A media condition inside `@media` that isn't widely available.
 - A CSS property value that isn't widely available or otherwise isn't enclosed in a `@supports` block (currently limited to identifiers only).
 - A CSS property function that isn't widely available.
+- A CSS pseudo-element or pseudo-class selector that isn't widely available.
 
 The data is provided via the [web-features](https://npmjs.com/package/web-features) package.
 
@@ -39,6 +40,18 @@ a {
 	width: abs(20% - 100px);
 }
 
+/* invalid - :has() is not widely available */
+h1:has(+ h2) {
+	margin: 0 0 0.25rem 0;
+}
+
+/* valid - @supports indicates you're choosing a limited availability selector */
+@supports selector(:has()) {
+	h1:has(+ h2) {
+		margin: 0 0 0.25rem 0;
+	}
+}
+
 /* invalid - device-posture is not widely available */
 @media (device-posture: folded) {
 	a {

src/rules/require-baseline.js

@@ -15,6 +15,7 @@ import {
 	atRules,
 	mediaConditions,
 	types,
+	selectors,
 } from "../data/baseline-data.js";
 import { namedColors } from "../data/colors.js";
 
@@ -123,6 +124,12 @@ class SupportsRule {
 	 */
 	#properties = new Map();
 
+	/**
+	 * The selectors supported by this rule.
+	 * @type {Set<string>}
+	 */
+	#selectors = new Set();
+
 	/**
 	 * Adds a property to the rule.
 	 * @param {string} property The name of the property.
@@ -218,6 +225,24 @@ class SupportsRule {
 
 		return supportedProperty.hasFunctions();
 	}
+
+	/**
+	 * Adds a selector to the rule.
+	 * @param {string} selector The name of the selector.
+	 * @returns {void}
+	 */
+	addSelector(selector) {
+		this.#selectors.add(selector);
+	}
+
+	/**
+	 * Determines if the rule supports a selector.
+	 * @param {string} selector The name of the selector.
+	 * @returns {boolean} `true` if the selector is supported, `false` if not.
+	 */
+	hasSelector(selector) {
+		return this.#selectors.has(selector);
+	}
 }
 
 /**
@@ -303,6 +328,15 @@ class SupportsRules {
 	hasPropertyFunctions(property) {
 		return this.#rules.some(rule => rule.hasFunctions(property));
 	}
+
+	/**
+	 * Determines if any rule supports a selector.
+	 * @param {string} selector The name of the selector.
+	 * @returns {boolean} `true` if any rule supports the selector, `false` if not.
+	 */
+	hasSelector(selector) {
+		return this.#rules.some(rule => rule.hasSelector(selector));
+	}
 }
 
 //-----------------------------------------------------------------------------
@@ -347,6 +381,8 @@ export default {
 				"Type '{{type}}' is not a {{availability}} available baseline feature.",
 			notBaselineMediaCondition:
 				"Media condition '{{condition}}' is not a {{availability}} available baseline feature.",
+			notBaselineSelector:
+				"Selector '{{selector}}' is not a {{availability}} available baseline feature.",
 		},
 	},
 
@@ -459,6 +495,16 @@ export default {
 
 						continue;
 					}
+
+					if (
+						conditionChild.type === "FeatureFunction" &&
+						conditionChild.feature === "selector"
+					) {
+						for (const selectorChild of conditionChild.value
+							.children) {
+							supportsRule.addSelector(selectorChild.name);
+						}
+					}
 				}
 			},
 
@@ -625,6 +671,53 @@ export default {
 					});
 				}
 			},
+
+			Selector(node) {
+				for (const child of node.children) {
+					const selector = child.name;
+
+					if (!selectors.has(selector)) {
+						continue;
+					}
+
+					// if the selector has been tested in a @supports rule, don't check it
+					if (supportsRules.hasSelector(selector)) {
+						continue;
+					}
+
+					const selectorLevel = selectors.get(selector);
+
+					if (selectorLevel < baselineLevel) {
+						const loc = child.loc;
+
+						// some selectors are prefixed with the : or :: symbols
+						let prefixSymbolLength = 0;
+						if (child.type === "PseudoClassSelector") {
+							prefixSymbolLength = 1;
+						} else if (child.type === "PseudoElementSelector") {
+							prefixSymbolLength = 2;
+						}
+
+						context.report({
+							loc: {
+								start: loc.start,
+								end: {
+									line: loc.start.line,
+									column:
+										loc.start.column +
+										selector.length +
+										prefixSymbolLength,
+								},
+							},
+							messageId: "notBaselineSelector",
+							data: {
+								selector,
+								availability,
+							},
+						});
+					}
+				}
+			},
 		};
 	},
 };

tests/rules/require-baseline.test.js

@@ -57,6 +57,9 @@ ruleTester.run("require-baseline", rule, {
 		`@supports (width: abs(20% - 100px)) {
 			a { width: abs(20% - 100px); }
 		}`,
+		`@supports selector(:has()) {
+				h1:has(+ h2) { color: red; }
+		}`,
 		"div { cursor: pointer; }",
 		{
 			code: `@property --foo {
@@ -340,5 +343,59 @@ ruleTester.run("require-baseline", rule, {
 				},
 			],
 		},
+		{
+			code: "h1:has(+ h2) { margin: 0 0 0.25rem 0; }",
+			errors: [
+				{
+					messageId: "notBaselineSelector",
+					data: {
+						selector: "has",
+						availability: "widely",
+					},
+					line: 1,
+					column: 3,
+					endLine: 1,
+					endColumn: 7,
+				},
+			],
+		},
+		{
+			code: `@supports selector(:has()) {}
+
+			@supports (color: red) {
+				h1:has(+ h2) {
+					color: red;
+				}
+			}`,
+			errors: [
+				{
+					messageId: "notBaselineSelector",
+					data: {
+						selector: "has",
+						availability: "widely",
+					},
+					line: 4,
+					column: 7,
+					endLine: 4,
+					endColumn: 11,
+				},
+			],
+		},
+		{
+			code: "details::details-content { background-color: #a29bfe; }",
+			errors: [
+				{
+					messageId: "notBaselineSelector",
+					data: {
+						selector: "details-content",
+						availability: "widely",
+					},
+					line: 1,
+					column: 8,
+					endLine: 1,
+					endColumn: 25,
+				},
+			],
+		},
 	],
 });