Allow HTML comments containing "--" even though it is not allowed by the CommonMark specification because it is allowed by the HTML specification and it happens in real life (fixes #996).

demo/markdownlint-browser.js

@@ -1217,8 +1217,16 @@ function isHtmlFlowComment(token) {
     type = token.type;
   if (type === "htmlFlow" && text.startsWith("<!--") && text.endsWith("-->")) {
     var comment = text.slice(4, -3);
-    return !comment.startsWith(">") && !comment.startsWith("->") && !comment.endsWith("-") && !comment.includes("--");
+    return !comment.startsWith(">") && !comment.startsWith("->") && !comment.endsWith("-")
+    // The following condition from the CommonMark specification is commented
+    // to avoid parsing HTML comments that include "--" because that is NOT a
+    // condition of the HTML specification.
+    // https://spec.commonmark.org/0.30/#raw-html
+    // https://html.spec.whatwg.org/multipage/syntax.html#comments
+    // && !comment.includes("--")
+    ;
   }
+
   return false;
 }
 

helpers/micromark.cjs

@@ -42,8 +42,13 @@ function isHtmlFlowComment(token) {
     return (
       !comment.startsWith(">") &&
       !comment.startsWith("->") &&
-      !comment.endsWith("-") &&
+      !comment.endsWith("-")
-      !comment.includes("--")
+      // The following condition from the CommonMark specification is commented
+      // to avoid parsing HTML comments that include "--" because that is NOT a
+      // condition of the HTML specification.
+      // https://spec.commonmark.org/0.30/#raw-html
+      // https://html.spec.whatwg.org/multipage/syntax.html#comments
+      // && !comment.includes("--")
     );
   }
   return false;

test/front-matter-yaml-in-html-comment.md

@@ -0,0 +1,14 @@
+<!--
+---
+title: Front Matter YAML in HTML Comment
+not-md035: |-
+  ***
+  * * *
+description: |-
+  This document has YAML front matter inside an HTML comment.
+---
+-->
+
+# Front Matter YAML in HTML Comment
+
+Text text text

test/snapshots/markdownlint-test-scenarios.js.md

@@ -13866,6 +13866,29 @@ Generated by [AVA](https://avajs.dev).
       `,
     }
 
+## front-matter-yaml-in-html-comment.md
+
+> Snapshot 1
+
+    {
+      errors: [],
+      fixed: `<!--␊
+      ---␊
+      title: Front Matter YAML in HTML Comment␊
+      not-md035: |-␊
+        ***␊
+        * * *␊
+      description: |-␊
+        This document has YAML front matter inside an HTML comment.␊
+      ---␊
+      -->␊
+      ␊
+      # Front Matter YAML in HTML Comment␊
+      ␊
+      Text text text␊
+      `,
+    }
+
 ## front-matter.md
 
 > Snapshot 1