diff --git a/src/w3c/conformance.js b/src/w3c/conformance.js
index 63c09c04c5..e0f2744512 100644
--- a/src/w3c/conformance.js
+++ b/src/w3c/conformance.js
@@ -1,7 +1,12 @@
 // @ts-check
 // Module w3c/conformance
 // Handle the conformance section properly.
-import { getIntlData, htmlJoinAnd, showWarning } from "../core/utils.js";
+import {
+  getIntlData,
+  htmlJoinAnd,
+  showError,
+  showWarning,
+} from "../core/utils.js";
 import { html } from "../core/import-maps.js";
 import { renderInlineCitation } from "../core/render-biblio.js";
 import { rfc2119Usage } from "../core/inlines.js";
@@ -75,9 +80,18 @@ function processConformance(conformance, conf) {
 }
 
 export function run(conf) {
+  /** @type {HTMLElement | null} */
   const conformance = document.querySelector("section#conformance");
-  if (conformance && !conformance.classList.contains("override")) {
-    processConformance(conformance, conf);
+  if (conformance) {
+    if (conformance.classList.contains("informative")) {
+      const msg = "The conformance section cannot be marked as informative.";
+      const hint =
+        'Remove `class="informative"` from `<section id="conformance">`. Conformance sections are normative by definition.';
+      showError(msg, name, { hint, elements: [conformance] });
+    }
+    if (!conformance.classList.contains("override")) {
+      processConformance(conformance, conf);
+    }
   }
   // Warn when there are RFC2119/RFC8174 keywords, but not conformance section
   if (!conformance && Object.keys(rfc2119Usage).length) {
diff --git a/tests/spec/w3c/conformance-spec.js b/tests/spec/w3c/conformance-spec.js
index c5cfcc93b2..03d949e432 100644
--- a/tests/spec/w3c/conformance-spec.js
+++ b/tests/spec/w3c/conformance-spec.js
@@ -64,6 +64,25 @@ describe("W3C — Conformance", () => {
     expect(doc.querySelectorAll("#conformance .rfc2119")).toHaveSize(0);
   });
 
+  it("errors when the conformance section is marked informative", async () => {
+    const body = `
+      <section id="conformance" class="informative">
+        <p>CONFORMANCE</p>
+      </section>
+      <section>
+        <h2>my section</h2>
+        <p>MUST be tested.</p>
+      </section>
+    `;
+    const doc = await makeRSDoc(makeStandardOps({}, body));
+    const errors = doc.respec.errors.filter(
+      e => e.plugin === "w3c/conformance"
+    );
+    expect(errors).toHaveSize(1);
+    expect(errors[0].message).toContain("cannot be marked as informative");
+    expect(errors[0].hint).toContain("informative");
+  });
+
   it("allows conformance section to be completely overridden via .override css class", async () => {
     const body = `
       <section>