fix: allow both null and empty string when attribute allows empty values

elements/html5.json

@@ -25,6 +25,9 @@
       "name",
       "urn"
     ],
+    "attributes": {
+      "download": ["", "/.+/"]
+    },
     "permittedDescendants": [{ "exclude": "@interactive" }]
   },
 

src/meta/validator.spec.ts

@@ -589,6 +589,18 @@ describe("Meta validator", () => {
 			).toBeTruthy();
 		});
 
+		it("should consider empty value as either null or empty string", () => {
+			const rules = {
+				foo: [""] as string[],
+			};
+			expect(
+				Validator.validateAttribute(new Attribute("foo", null), rules)
+			).toBeTruthy();
+			expect(
+				Validator.validateAttribute(new Attribute("foo", ""), rules)
+			).toBeTruthy();
+		});
+
 		it("should normalize boolean attributes", () => {
 			const rules = {
 				foo: [] as string[],

src/meta/validator.ts

@@ -183,10 +183,17 @@ export class Validator {
 		if (value instanceof DynamicValue) {
 			return true;
 		}
+		const empty = value === null || value === "";
 
 		/* consider an empty array as being a boolean attribute */
 		if (rule.length === 0) {
-			return value === null || value === "" || value === attr.key;
+			return empty || value === attr.key;
+		}
+
+		/* if the empty string is present allow both "" and null
+		 * (boolean-attribute-style will regulate which is allowed) */
+		if (rule.includes("") && empty) {
+			return true;
 		}
 
 		return rule.some((entry: string | RegExp) => {

src/rules/attribute-allowed-values.spec.ts

@@ -64,6 +64,21 @@ describe("rule attribute-allowed-values", () => {
 		expect(report).toBeValid();
 	});
 
+	it("should not report error when element allows empty value and attribute is null", () => {
+		const report = htmlvalidate.validateString("<a download>");
+		expect(report).toBeValid();
+	});
+
+	it("should not report error when element allows empty value and attribute is empty string", () => {
+		const report = htmlvalidate.validateString('<a download="">');
+		expect(report).toBeValid();
+	});
+
+	it("should not report error when element allows empty and other values and attribute is non-empty string", () => {
+		const report = htmlvalidate.validateString('<a download="foobar">');
+		expect(report).toBeValid();
+	});
+
 	it("smoketest", () => {
 		const report = htmlvalidate.validateFile(
 			"test-files/rules/attribute-allowed-values.html"

test-files/elements/a-valid.html

@@ -9,3 +9,8 @@
 <!-- should be transparent -->
 <div><a><div>foo</div></a></div>
 <span><a><span>foo</span></a></span>
+
+<!-- should allow download attribute with or without value -->
+<a download>foo</a>
+<a download="">foo</a>
+<a download="foo.txtr">foo</a>