diff --git a/src/__tests__/attributeValues.mjs b/src/__tests__/attributeValues.mjs
new file mode 100644
index 0000000..f4d9e07
--- /dev/null
+++ b/src/__tests__/attributeValues.mjs
@@ -0,0 +1,77 @@
+import assert from "node:assert/strict";
+import test from "./util/runner.mjs";
+import parser from "../../dist/index.js";
+import postcss from "postcss";
+
+const message = "Expected an attribute value.";
+const operators = ["=", "~=", "|=", "^=", "$=", "*="];
+
+for (const operator of operators) {
+  test(`missing attribute value after ${operator}`, (t) => {
+    const input = `[href${operator}]`;
+    t.throws(() => parser().processSync(input), { message });
+    t.throws(() => parser().astSync(input), { message });
+  });
+
+  test(`quoted empty attribute value after ${operator}`, (t) => {
+    for (const quote of ['"', "'"]) {
+      const input = `[href${operator}${quote}${quote} i]`;
+      const attribute = parser().astSync(input).first.first;
+      t.is(attribute.value, "");
+      t.is(attribute.operator, operator);
+      t.is(attribute.insensitive, true);
+      t.is(parser().processSync(input), input);
+    }
+  });
+
+  test(`unquoted attribute value after ${operator}`, (t) => {
+    const input = `[href${operator}example]`;
+    t.is(parser().astSync(input).first.first.value, "example");
+    t.is(parser().processSync(input), input);
+  });
+}
+
+for (const input of [
+  "[href= ]",
+  "[href=\n\t ]",
+  "[href=/**/]",
+  "[href= /* value */ ]",
+  "[ns|href=]",
+  "[*|href=]",
+  "[|href=]",
+  "[ ns | href = ]",
+]) {
+  test(`missing attribute value in ${JSON.stringify(input)}`, (t) => {
+    t.throws(() => parser().processSync(input), { message });
+    t.throws(() => parser().processSync(input, { lossless: false }), { message });
+  });
+}
+
+test("missing attribute value rejects asynchronous processing", async () => {
+  await assert.rejects(parser().process("[href=]"), { message });
+  await assert.rejects(parser().ast("[href=]"), { message });
+});
+
+test("missing attribute value reports the closing bracket location", (t) => {
+  const rule = postcss.parse("a,\n  [href=] {}").first;
+  const error = t.throws(() => parser().processSync(rule));
+  t.is(error.name, "CssSyntaxError");
+  t.is(error.reason, message);
+  t.is(error.line, 2);
+  t.is(error.column, 9);
+});
+
+test("presence and namespaced quoted-empty attributes remain valid", (t) => {
+  for (const input of [
+    "[href]",
+    "[ns|href]",
+    '[ns|href=""]',
+    "[*|href='']",
+    '[|href="" i]',
+    '[ ns | href = "" ]',
+    '[href=/**/""/* value */]',
+  ]) {
+    t.is(parser().processSync(input), input);
+  }
+  t.is(parser().processSync('[ ns | href = "" ]', { lossless: false }), '[ns|href=""]');
+});
diff --git a/src/parser.js b/src/parser.js
index 22d5e69..ee8f41b 100644
--- a/src/parser.js
+++ b/src/parser.js
@@ -444,6 +444,9 @@ export default class Parser {
       }
       pos++;
     }
+    if (node.operator && node.value === undefined) {
+      return this.expected("attribute value", this.currToken[TOKEN.START_POS]);
+    }
     unescapeProp(node, "attribute");
     unescapeProp(node, "namespace");
     this.newNode(new Attribute(node));