From e0078364148fbfae61e9b85288a653812b98228f Mon Sep 17 00:00:00 2001
From: LuLaValva <lukelavalva@gmail.com>
Date: Fri, 14 Aug 2026 17:08:29 -0700
Subject: [PATCH 1/4] fix(compiler): reject an html comment in a tag's
 attributes
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

An `<!-- … -->` written where an attribute belongs either surfaced
htmljs-parser's unrelated "Unexpected types" message or, once the `<!--`
landed after an attribute value, was swallowed into that expression and
compiled silently into mangled attributes.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
---
 .changeset/loud-attr-html-comments.md         |  6 ++
 agent-feedback/dx.md                          |  6 ++
 packages/compiler/src/babel-plugin/parser.js  | 62 ++++++++++++++++++-
 packages/runtime-tags/cheatsheet.md           |  1 +
 .../__snapshots__/error-compile-dom.debug.txt |  5 ++
 .../__snapshots__/error-compile-dom.txt       |  5 ++
 .../error-compile-html.debug.txt              |  5 ++
 .../__snapshots__/error-compile-html.txt      |  5 ++
 .../template.marko                            |  1 +
 .../error-html-comment-after-attr/test.ts     |  5 ++
 .../__snapshots__/error-compile-dom.debug.txt |  8 +++
 .../__snapshots__/error-compile-dom.txt       |  8 +++
 .../error-compile-html.debug.txt              |  8 +++
 .../__snapshots__/error-compile-html.txt      |  8 +++
 .../template.marko                            |  4 ++
 .../error-html-comment-in-attrs/test.ts       |  5 ++
 16 files changed, 140 insertions(+), 2 deletions(-)
 create mode 100644 .changeset/loud-attr-html-comments.md
 create mode 100644 packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-dom.debug.txt
 create mode 100644 packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-dom.txt
 create mode 100644 packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-html.debug.txt
 create mode 100644 packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-html.txt
 create mode 100644 packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/template.marko
 create mode 100644 packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/test.ts
 create mode 100644 packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-dom.debug.txt
 create mode 100644 packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-dom.txt
 create mode 100644 packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-html.debug.txt
 create mode 100644 packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-html.txt
 create mode 100644 packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/template.marko
 create mode 100644 packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/test.ts

diff --git a/.changeset/loud-attr-html-comments.md b/.changeset/loud-attr-html-comments.md
new file mode 100644
index 00000000000..a89fbb73b6a
--- /dev/null
+++ b/.changeset/loud-attr-html-comments.md
@@ -0,0 +1,6 @@
+---
+"@marko/compiler": patch
+"@marko/runtime-tags": patch
+---
+
+Report an HTML comment written inside a tag's attributes as its own compile error. Previously `<div <!-- note --> class="a">` either surfaced an unrelated "Unexpected types" parse error or, when the `<!--` landed after an attribute value, silently compiled into a mangled expression.
diff --git a/agent-feedback/dx.md b/agent-feedback/dx.md
index 1739b89431f..295a9a5cb7f 100644
--- a/agent-feedback/dx.md
+++ b/agent-feedback/dx.md
@@ -85,3 +85,9 @@ After resume `$global` holds only `{runtimeId, renderId}` unless a key is enable
 `packages/runtime-tags/src/translator/core/script.ts` › default export | 2026-08-10 | impact:high | effort:med
 
 A `<script>` re-runs when any binding it references changes, and an assignment like `items = items.filter(...)` inside that same script produces a new array identity on every run — so the script re-triggers itself forever. In a real app the loop sat inside a data loader (`openIds = openIds.filter(id => list.some(...))` after a fetch), yielding an unbounded three-requests-per-cycle fetch loop that only surfaced as the browser's `ERR_INSUFFICIENT_RESOURCES`, far from the cause; the fix was guarding the assignment behind a length comparison to preserve identity. The hazard is statically visible: the script's body both reads a binding and unconditionally assigns it an expression guaranteed to be a fresh object identity (array/object literal, `.filter`/`.map`/`.concat`/spread). Warn on that shape, pointing at identity-preserving guards or `<const>`. Re-verify: mount `<let/items=[[]]>` with `<script>items = items.filter(Boolean); console.count("run")</script>` in jsdom — the counter never stops.
+
+## Flatten `.mocharc.json`'s `node-option` array so serial runs get `--experimental-vm-modules`
+
+`.mocharc.json` › `node-option` | 2026-08-14 | impact:med | effort:low
+
+The documented scoped-run command (root `CLAUDE.md`, `pnpm test -- --grep "runtime-tags/translator <fixture> "`) drops every entry in `.mocharc.json`'s `node-option` array on Node 26 + mocha 11, so any fixture with an `ssr` case fails with `TypeError: vm.SourceTextModule is not a constructor` and the suppressed `MODULE_TYPELESS_PACKAGE_JSON` warnings reappear. `.mocharc.parallel.json` spells the same flags as top-level booleans (`"experimental-vm-modules": true`) and is honored, which is why `pnpm run test:parallel` and CI pass and only the serial path breaks — an agent iterating on one fixture sees a failure that has nothing to do with its change. Rewrite the array as top-level keys to match the parallel config. Re-verify: `pnpm exec mocha --config .mocharc.all.json --grep "runtime-tags/translator html-comment-counter"` fails, and the same command under `NODE_OPTIONS="--experimental-vm-modules"` passes.
diff --git a/packages/compiler/src/babel-plugin/parser.js b/packages/compiler/src/babel-plugin/parser.js
index f3a58ff70db..1f7d4653c57 100644
--- a/packages/compiler/src/babel-plugin/parser.js
+++ b/packages/compiler/src/babel-plugin/parser.js
@@ -36,6 +36,8 @@ const withWrappedAttrValueHint = (file, part, rawValue, node) => {
   }
   return node;
 };
+const htmlCommentInAttrsError =
+  "HTML comments are not supported in a tag's attributes. Use a JavaScript comment (`// …` or `/* … */`), or move the `<!-- … -->` outside the tag.";
 const emptyRange = (part) => part.start === part.end;
 const isAttrTag = (tag) => tag.name.value?.[0] === "@";
 const isStatementTag = (tag) => tag.tagDef?.parseOptions?.statement;
@@ -146,11 +148,17 @@ export function parseMarko(file) {
 
   const parser = createParser({
     onError(part) {
+      // htmljs-parser reads the `<` of a `<!--` sitting where an attribute
+      // belongs as tag type arguments, and reports that instead.
+      const isHTMLComment =
+        part.start > 0 && code.startsWith("<!--", part.start - 1);
       const err = buildCodeFrameError(
         file.opts.filename,
         file.code,
-        locationAt(part),
-        part.message,
+        locationAt(
+          isHTMLComment ? { start: part.start - 1, end: part.start + 3 } : part,
+        ),
+        isHTMLComment ? htmlCommentInAttrsError : part.message,
       );
 
       if (!file.___hasParseErrors) {
@@ -504,6 +512,25 @@ export function parseMarko(file) {
       const { node } = currentTag;
       const { attributes } = node;
       const parseOptions = node.tagDef?.parseOptions;
+      // A `<!--` anywhere else in the open tag was swallowed into an attribute
+      // expression, so nothing downstream can still recognize it as a comment.
+      const htmlCommentIndex = findHTMLCommentInAttrs(
+        code,
+        node.name.end,
+        part.start,
+      );
+      if (~htmlCommentIndex) {
+        throw buildCodeFrameError(
+          file.opts.filename,
+          file.code,
+          locationAt({
+            start: htmlCommentIndex,
+            end: htmlCommentIndex + 4,
+          }),
+          htmlCommentInAttrsError,
+        );
+      }
+
       endAttr();
 
       if (currentShorthandClassNames) {
@@ -721,6 +748,37 @@ function sortByStart(a, b) {
   return a.start - b.start;
 }
 
+// Strings and JS comments are the only places between a tag name and the end of
+// its open tag where a literal `<!--` is the author's text rather than a mistake.
+function findHTMLCommentInAttrs(code, start, end) {
+  for (let i = start; i < end; i++) {
+    switch (code[i]) {
+      case '"':
+      case "'":
+      case "`": {
+        const quote = code[i];
+        while (++i < end && code[i] !== quote) {
+          if (code[i] === "\\") i++;
+        }
+        break;
+      }
+      case "/":
+        if (code[i + 1] === "/") {
+          while (++i < end && code[i] !== "\n");
+        } else if (code[i + 1] === "*") {
+          while (++i < end && !(code[i] === "*" && code[i + 1] === "/"));
+          i++;
+        }
+        break;
+      case "<":
+        if (code.startsWith("<!--", i)) return i;
+        break;
+    }
+  }
+
+  return -1;
+}
+
 function templateElement(value, tail) {
   return t.templateElement({
     tail,
diff --git a/packages/runtime-tags/cheatsheet.md b/packages/runtime-tags/cheatsheet.md
index 517a6c1fac7..b2139649dc9 100644
--- a/packages/runtime-tags/cheatsheet.md
+++ b/packages/runtime-tags/cheatsheet.md
@@ -196,6 +196,7 @@ Each left-hand habit is an error or silently wrong.
 | ----------------------------------------------------------- | ------------------------------------------------------------------------------------ |
 | `disabled=n>=8` (hugging `>` in a value)                    | `disabled=n >= 8` or `disabled=(n >= 8)`; a hugging `>` silently closes the tag      |
 | `<let/s=new Set<string>()>` (type argument in a value)      | `<let/s=(new Set<string>())>`; a tag-var annotation fails type-check                 |
+| `<div <!-- note --> class="a">` (comment in the attributes) | `<div /* note */ class="a">`; `<!-- … -->` is markup, so it only belongs in a body   |
 | `{expr}` in markup, `className`, `key=`, `style={{...}}`    | `${expr}`, `class`, `by=` on `<for>`, `style={...}`                                  |
 | `onClick={() => ...}` / `@click` / `on-click("name")`       | `onClick() { ... }`                                                                  |
 | `const [x, setX] = useState()` / `state` / `class {}` block | `<let/x=0>` then `x = 1`                                                             |
diff --git a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-dom.debug.txt b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-dom.debug.txt
new file mode 100644
index 00000000000..79861b7364d
--- /dev/null
+++ b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-dom.debug.txt
@@ -0,0 +1,5 @@
+
+    at packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/template.marko:1:18
+    > 1 | <div class="foo" <!-- the label --> id="bar">Hello</div>
+        |                  ^^^^ HTML comments are not supported in a tag's attributes. Use a JavaScript comment (`// …` or `/* … */`), or move the `<!-- … -->` outside the tag.
+      2 |
\ No newline at end of file
diff --git a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-dom.txt b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-dom.txt
new file mode 100644
index 00000000000..79861b7364d
--- /dev/null
+++ b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-dom.txt
@@ -0,0 +1,5 @@
+
+    at packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/template.marko:1:18
+    > 1 | <div class="foo" <!-- the label --> id="bar">Hello</div>
+        |                  ^^^^ HTML comments are not supported in a tag's attributes. Use a JavaScript comment (`// …` or `/* … */`), or move the `<!-- … -->` outside the tag.
+      2 |
\ No newline at end of file
diff --git a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-html.debug.txt b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-html.debug.txt
new file mode 100644
index 00000000000..79861b7364d
--- /dev/null
+++ b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-html.debug.txt
@@ -0,0 +1,5 @@
+
+    at packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/template.marko:1:18
+    > 1 | <div class="foo" <!-- the label --> id="bar">Hello</div>
+        |                  ^^^^ HTML comments are not supported in a tag's attributes. Use a JavaScript comment (`// …` or `/* … */`), or move the `<!-- … -->` outside the tag.
+      2 |
\ No newline at end of file
diff --git a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-html.txt b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-html.txt
new file mode 100644
index 00000000000..79861b7364d
--- /dev/null
+++ b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-html.txt
@@ -0,0 +1,5 @@
+
+    at packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/template.marko:1:18
+    > 1 | <div class="foo" <!-- the label --> id="bar">Hello</div>
+        |                  ^^^^ HTML comments are not supported in a tag's attributes. Use a JavaScript comment (`// …` or `/* … */`), or move the `<!-- … -->` outside the tag.
+      2 |
\ No newline at end of file
diff --git a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/template.marko b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/template.marko
new file mode 100644
index 00000000000..d7a4575fb59
--- /dev/null
+++ b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/template.marko
@@ -0,0 +1 @@
+<div class="foo" <!-- the label --> id="bar">Hello</div>
diff --git a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/test.ts b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/test.ts
new file mode 100644
index 00000000000..87d511a1f39
--- /dev/null
+++ b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/test.ts
@@ -0,0 +1,5 @@
+import type { TestConfig } from "../../main.test";
+
+export const config: TestConfig = {
+  error_compiler: true,
+};
diff --git a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-dom.debug.txt b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-dom.debug.txt
new file mode 100644
index 00000000000..31fd71aa7e5
--- /dev/null
+++ b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-dom.debug.txt
@@ -0,0 +1,8 @@
+
+    at packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/template.marko:2:3
+      1 | <div
+    > 2 |   <!-- the label -->
+        |   ^^^^ HTML comments are not supported in a tag's attributes. Use a JavaScript comment (`// …` or `/* … */`), or move the `<!-- … -->` outside the tag.
+      3 |   class="foo"
+      4 | >Hello</div>
+      5 |
\ No newline at end of file
diff --git a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-dom.txt b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-dom.txt
new file mode 100644
index 00000000000..31fd71aa7e5
--- /dev/null
+++ b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-dom.txt
@@ -0,0 +1,8 @@
+
+    at packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/template.marko:2:3
+      1 | <div
+    > 2 |   <!-- the label -->
+        |   ^^^^ HTML comments are not supported in a tag's attributes. Use a JavaScript comment (`// …` or `/* … */`), or move the `<!-- … -->` outside the tag.
+      3 |   class="foo"
+      4 | >Hello</div>
+      5 |
\ No newline at end of file
diff --git a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-html.debug.txt b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-html.debug.txt
new file mode 100644
index 00000000000..31fd71aa7e5
--- /dev/null
+++ b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-html.debug.txt
@@ -0,0 +1,8 @@
+
+    at packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/template.marko:2:3
+      1 | <div
+    > 2 |   <!-- the label -->
+        |   ^^^^ HTML comments are not supported in a tag's attributes. Use a JavaScript comment (`// …` or `/* … */`), or move the `<!-- … -->` outside the tag.
+      3 |   class="foo"
+      4 | >Hello</div>
+      5 |
\ No newline at end of file
diff --git a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-html.txt b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-html.txt
new file mode 100644
index 00000000000..31fd71aa7e5
--- /dev/null
+++ b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-html.txt
@@ -0,0 +1,8 @@
+
+    at packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/template.marko:2:3
+      1 | <div
+    > 2 |   <!-- the label -->
+        |   ^^^^ HTML comments are not supported in a tag's attributes. Use a JavaScript comment (`// …` or `/* … */`), or move the `<!-- … -->` outside the tag.
+      3 |   class="foo"
+      4 | >Hello</div>
+      5 |
\ No newline at end of file
diff --git a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/template.marko b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/template.marko
new file mode 100644
index 00000000000..af43a205c7f
--- /dev/null
+++ b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/template.marko
@@ -0,0 +1,4 @@
+<div
+  <!-- the label -->
+  class="foo"
+>Hello</div>
diff --git a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/test.ts b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/test.ts
new file mode 100644
index 00000000000..87d511a1f39
--- /dev/null
+++ b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/test.ts
@@ -0,0 +1,5 @@
+import type { TestConfig } from "../../main.test";
+
+export const config: TestConfig = {
+  error_compiler: true,
+};

From a34b166d27338be21c824034b2e8af72ca95c750 Mon Sep 17 00:00:00 2001
From: LuLaValva <lukelavalva@gmail.com>
Date: Fri, 14 Aug 2026 17:17:49 -0700
Subject: [PATCH 2/4] refactor(compiler): fold the html comment error into one
 throw helper

Both detection sites now share `throwHTMLCommentInAttrs`, which restores
`onError` to its original shape and drops the comments that stood in for
the naming.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
---
 packages/compiler/src/babel-plugin/parser.js | 51 +++++++++-----------
 1 file changed, 23 insertions(+), 28 deletions(-)

diff --git a/packages/compiler/src/babel-plugin/parser.js b/packages/compiler/src/babel-plugin/parser.js
index 1f7d4653c57..dfaf121a599 100644
--- a/packages/compiler/src/babel-plugin/parser.js
+++ b/packages/compiler/src/babel-plugin/parser.js
@@ -36,8 +36,7 @@ const withWrappedAttrValueHint = (file, part, rawValue, node) => {
   }
   return node;
 };
-const htmlCommentInAttrsError =
-  "HTML comments are not supported in a tag's attributes. Use a JavaScript comment (`// …` or `/* … */`), or move the `<!-- … -->` outside the tag.";
+const htmlCommentOpen = "<!--";
 const emptyRange = (part) => part.start === part.end;
 const isAttrTag = (tag) => tag.name.value?.[0] === "@";
 const isStatementTag = (tag) => tag.tagDef?.parseOptions?.statement;
@@ -113,6 +112,14 @@ export function parseMarko(file) {
       currentAttr = undefined;
     }
   };
+  const throwHTMLCommentInAttrs = (start) => {
+    throw buildCodeFrameError(
+      file.opts.filename,
+      file.code,
+      locationAt({ start, end: start + htmlCommentOpen.length }),
+      "HTML comments are not supported in a tag's attributes. Use a JavaScript comment (`// …` or `/* … */`), or move the `<!-- … -->` outside the tag.",
+    );
+  };
   const parseTemplateString = ({ quasis, expressions }) => {
     switch (expressions.length) {
       case 0: {
@@ -148,17 +155,19 @@ export function parseMarko(file) {
 
   const parser = createParser({
     onError(part) {
-      // htmljs-parser reads the `<` of a `<!--` sitting where an attribute
-      // belongs as tag type arguments, and reports that instead.
-      const isHTMLComment =
-        part.start > 0 && code.startsWith("<!--", part.start - 1);
+      const htmlCommentIndex = part.start - 1;
+      if (
+        htmlCommentIndex >= 0 &&
+        code.startsWith(htmlCommentOpen, htmlCommentIndex)
+      ) {
+        throwHTMLCommentInAttrs(htmlCommentIndex);
+      }
+
       const err = buildCodeFrameError(
         file.opts.filename,
         file.code,
-        locationAt(
-          isHTMLComment ? { start: part.start - 1, end: part.start + 3 } : part,
-        ),
-        isHTMLComment ? htmlCommentInAttrsError : part.message,
+        locationAt(part),
+        part.message,
       );
 
       if (!file.___hasParseErrors) {
@@ -512,24 +521,12 @@ export function parseMarko(file) {
       const { node } = currentTag;
       const { attributes } = node;
       const parseOptions = node.tagDef?.parseOptions;
-      // A `<!--` anywhere else in the open tag was swallowed into an attribute
-      // expression, so nothing downstream can still recognize it as a comment.
       const htmlCommentIndex = findHTMLCommentInAttrs(
         code,
         node.name.end,
         part.start,
       );
-      if (~htmlCommentIndex) {
-        throw buildCodeFrameError(
-          file.opts.filename,
-          file.code,
-          locationAt({
-            start: htmlCommentIndex,
-            end: htmlCommentIndex + 4,
-          }),
-          htmlCommentInAttrsError,
-        );
-      }
+      if (~htmlCommentIndex) throwHTMLCommentInAttrs(htmlCommentIndex);
 
       endAttr();
 
@@ -748,11 +745,12 @@ function sortByStart(a, b) {
   return a.start - b.start;
 }
 
-// Strings and JS comments are the only places between a tag name and the end of
-// its open tag where a literal `<!--` is the author's text rather than a mistake.
 function findHTMLCommentInAttrs(code, start, end) {
   for (let i = start; i < end; i++) {
     switch (code[i]) {
+      case "<":
+        if (code.startsWith(htmlCommentOpen, i)) return i;
+        break;
       case '"':
       case "'":
       case "`": {
@@ -770,9 +768,6 @@ function findHTMLCommentInAttrs(code, start, end) {
           i++;
         }
         break;
-      case "<":
-        if (code.startsWith("<!--", i)) return i;
-        break;
     }
   }
 

From 4442ef59db1639ff80be5d9d86adf9950a756cc8 Mon Sep 17 00:00:00 2001
From: LuLaValva <lukelavalva@gmail.com>
Date: Fri, 14 Aug 2026 17:29:06 -0700
Subject: [PATCH 3/4] docs(agent-feedback): record the regex blind spot in the
 attr comment scan

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
---
 agent-feedback/bugs.md | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/agent-feedback/bugs.md b/agent-feedback/bugs.md
index a78000f2637..039befb55d9 100644
--- a/agent-feedback/bugs.md
+++ b/agent-feedback/bugs.md
@@ -58,3 +58,9 @@ and after resume.
 `packages/runtime-class/src/runtime/helpers/dynamic-tag.js` › `addTagsEvents` | 2026-08-05 | impact:med | effort:high
 
 A _split_ Class component (one with a `component-browser.js`, so `FLAG_WILL_RERENDER_IN_BROWSER` is unset) that passes an inline function to a Tags child — `<tags-pinger onPing(count) { component.handlePing(count) }/>` — resumes with a permanently dead handler: `registerClassFunctions` serializes it as the compat noop, and unlike a rerendering parent the split parent never re-feeds a live one. The string form `onPing("handlePing")` works, because `addTagsEvents` serializes `[CLASS_EVENT_MARKER, componentId, method]` and `runtime-dom.js`'s `setClassEventResolver` revives it by name; an inline closure has no name to bridge, so nothing can revive it. A real fix needs the Marko 5 translator to give each inline handler an id reachable from the browser bundle so it can serialize a marker like the named form; failing that, a compile-time or `MARKO_DEBUG` error on this exact shape beats silently dropping events. Re-verify: copy `interop-event-split-class-to-tags` and replace `onPing("handlePing")` in `components/class-host/index.marko` with an inline body calling `component.handlePing(count)` — the fixture harness reports `Snapshot conflict: "render.debug.md" was written with different content by two tests`, because CSR updates `#class` and resumed SSR does not.
+
+## Skip regular expression literals when scanning a tag's attributes for `<!--`
+
+`packages/compiler/src/babel-plugin/parser.js` › `findHTMLCommentInAttrs` | 2026-08-14 | impact:low | effort:med
+
+`findHTMLCommentInAttrs` skips strings and JS comments but not regular expression literals, so an attribute value whose regex contains the sequence is rejected outright: `<div hidden=/<!--/.test(s) />` compiled before the scan was added and now fails with "HTML comments are not supported in a tag's attributes". Telling a regex from division needs the previous significant token plus character-class handling (`/[/]/`), which is why it was left out, but this is valid code the compiler now refuses. Deleting the scan is the better end state: htmljs-parser's `checkForOperators` already refuses to continue an attribute expression across `</`, so `<!--` could join it with its own error code and catch the orphan `-->` case too — though marko-js/htmljs-parser#170 raised this exact scenario and was closed as "complicates things more than it is worth", so it needs a fresh case made for the error rather than for emitting the comment. Re-verify: `compileSync('<div hidden=/<!--/.test(s) />', "/t.marko", { output: "html" })` throws.

From e2b3b11f70462748736e9768462a640b1e266ac9 Mon Sep 17 00:00:00 2001
From: LuLaValva <lukelavalva@gmail.com>
Date: Sat, 15 Aug 2026 08:13:09 -0700
Subject: [PATCH 4/4] revert(compiler): drop the attr html comment scan for the
 parser fix

htmljs-parser now reports an `<!--` inside an open tag itself
(marko-js/htmljs-parser#239), from the tokenizer states rather than a
source scan, so it also gets the regular expression literal case right.
Only the cheat sheet row stays.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
---
 .changeset/loud-attr-html-comments.md         |  3 +-
 agent-feedback/bugs.md                        |  6 ---
 packages/compiler/src/babel-plugin/parser.js  | 53 -------------------
 .../__snapshots__/error-compile-dom.debug.txt |  5 --
 .../__snapshots__/error-compile-dom.txt       |  5 --
 .../error-compile-html.debug.txt              |  5 --
 .../__snapshots__/error-compile-html.txt      |  5 --
 .../template.marko                            |  1 -
 .../error-html-comment-after-attr/test.ts     |  5 --
 .../__snapshots__/error-compile-dom.debug.txt |  8 ---
 .../__snapshots__/error-compile-dom.txt       |  8 ---
 .../error-compile-html.debug.txt              |  8 ---
 .../__snapshots__/error-compile-html.txt      |  8 ---
 .../template.marko                            |  4 --
 .../error-html-comment-in-attrs/test.ts       |  5 --
 15 files changed, 1 insertion(+), 128 deletions(-)
 delete mode 100644 packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-dom.debug.txt
 delete mode 100644 packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-dom.txt
 delete mode 100644 packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-html.debug.txt
 delete mode 100644 packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-html.txt
 delete mode 100644 packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/template.marko
 delete mode 100644 packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/test.ts
 delete mode 100644 packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-dom.debug.txt
 delete mode 100644 packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-dom.txt
 delete mode 100644 packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-html.debug.txt
 delete mode 100644 packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-html.txt
 delete mode 100644 packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/template.marko
 delete mode 100644 packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/test.ts

diff --git a/.changeset/loud-attr-html-comments.md b/.changeset/loud-attr-html-comments.md
index a89fbb73b6a..6c2595b84ed 100644
--- a/.changeset/loud-attr-html-comments.md
+++ b/.changeset/loud-attr-html-comments.md
@@ -1,6 +1,5 @@
 ---
-"@marko/compiler": patch
 "@marko/runtime-tags": patch
 ---
 
-Report an HTML comment written inside a tag's attributes as its own compile error. Previously `<div <!-- note --> class="a">` either surfaced an unrelated "Unexpected types" parse error or, when the `<!--` landed after an attribute value, silently compiled into a mangled expression.
+Add a cheat sheet `DON'T` row for an HTML comment written inside a tag's attributes, pointing at `/* … */` instead.
diff --git a/agent-feedback/bugs.md b/agent-feedback/bugs.md
index 039befb55d9..a78000f2637 100644
--- a/agent-feedback/bugs.md
+++ b/agent-feedback/bugs.md
@@ -58,9 +58,3 @@ and after resume.
 `packages/runtime-class/src/runtime/helpers/dynamic-tag.js` › `addTagsEvents` | 2026-08-05 | impact:med | effort:high
 
 A _split_ Class component (one with a `component-browser.js`, so `FLAG_WILL_RERENDER_IN_BROWSER` is unset) that passes an inline function to a Tags child — `<tags-pinger onPing(count) { component.handlePing(count) }/>` — resumes with a permanently dead handler: `registerClassFunctions` serializes it as the compat noop, and unlike a rerendering parent the split parent never re-feeds a live one. The string form `onPing("handlePing")` works, because `addTagsEvents` serializes `[CLASS_EVENT_MARKER, componentId, method]` and `runtime-dom.js`'s `setClassEventResolver` revives it by name; an inline closure has no name to bridge, so nothing can revive it. A real fix needs the Marko 5 translator to give each inline handler an id reachable from the browser bundle so it can serialize a marker like the named form; failing that, a compile-time or `MARKO_DEBUG` error on this exact shape beats silently dropping events. Re-verify: copy `interop-event-split-class-to-tags` and replace `onPing("handlePing")` in `components/class-host/index.marko` with an inline body calling `component.handlePing(count)` — the fixture harness reports `Snapshot conflict: "render.debug.md" was written with different content by two tests`, because CSR updates `#class` and resumed SSR does not.
-
-## Skip regular expression literals when scanning a tag's attributes for `<!--`
-
-`packages/compiler/src/babel-plugin/parser.js` › `findHTMLCommentInAttrs` | 2026-08-14 | impact:low | effort:med
-
-`findHTMLCommentInAttrs` skips strings and JS comments but not regular expression literals, so an attribute value whose regex contains the sequence is rejected outright: `<div hidden=/<!--/.test(s) />` compiled before the scan was added and now fails with "HTML comments are not supported in a tag's attributes". Telling a regex from division needs the previous significant token plus character-class handling (`/[/]/`), which is why it was left out, but this is valid code the compiler now refuses. Deleting the scan is the better end state: htmljs-parser's `checkForOperators` already refuses to continue an attribute expression across `</`, so `<!--` could join it with its own error code and catch the orphan `-->` case too — though marko-js/htmljs-parser#170 raised this exact scenario and was closed as "complicates things more than it is worth", so it needs a fresh case made for the error rather than for emitting the comment. Re-verify: `compileSync('<div hidden=/<!--/.test(s) />', "/t.marko", { output: "html" })` throws.
diff --git a/packages/compiler/src/babel-plugin/parser.js b/packages/compiler/src/babel-plugin/parser.js
index dfaf121a599..f3a58ff70db 100644
--- a/packages/compiler/src/babel-plugin/parser.js
+++ b/packages/compiler/src/babel-plugin/parser.js
@@ -36,7 +36,6 @@ const withWrappedAttrValueHint = (file, part, rawValue, node) => {
   }
   return node;
 };
-const htmlCommentOpen = "<!--";
 const emptyRange = (part) => part.start === part.end;
 const isAttrTag = (tag) => tag.name.value?.[0] === "@";
 const isStatementTag = (tag) => tag.tagDef?.parseOptions?.statement;
@@ -112,14 +111,6 @@ export function parseMarko(file) {
       currentAttr = undefined;
     }
   };
-  const throwHTMLCommentInAttrs = (start) => {
-    throw buildCodeFrameError(
-      file.opts.filename,
-      file.code,
-      locationAt({ start, end: start + htmlCommentOpen.length }),
-      "HTML comments are not supported in a tag's attributes. Use a JavaScript comment (`// …` or `/* … */`), or move the `<!-- … -->` outside the tag.",
-    );
-  };
   const parseTemplateString = ({ quasis, expressions }) => {
     switch (expressions.length) {
       case 0: {
@@ -155,14 +146,6 @@ export function parseMarko(file) {
 
   const parser = createParser({
     onError(part) {
-      const htmlCommentIndex = part.start - 1;
-      if (
-        htmlCommentIndex >= 0 &&
-        code.startsWith(htmlCommentOpen, htmlCommentIndex)
-      ) {
-        throwHTMLCommentInAttrs(htmlCommentIndex);
-      }
-
       const err = buildCodeFrameError(
         file.opts.filename,
         file.code,
@@ -521,13 +504,6 @@ export function parseMarko(file) {
       const { node } = currentTag;
       const { attributes } = node;
       const parseOptions = node.tagDef?.parseOptions;
-      const htmlCommentIndex = findHTMLCommentInAttrs(
-        code,
-        node.name.end,
-        part.start,
-      );
-      if (~htmlCommentIndex) throwHTMLCommentInAttrs(htmlCommentIndex);
-
       endAttr();
 
       if (currentShorthandClassNames) {
@@ -745,35 +721,6 @@ function sortByStart(a, b) {
   return a.start - b.start;
 }
 
-function findHTMLCommentInAttrs(code, start, end) {
-  for (let i = start; i < end; i++) {
-    switch (code[i]) {
-      case "<":
-        if (code.startsWith(htmlCommentOpen, i)) return i;
-        break;
-      case '"':
-      case "'":
-      case "`": {
-        const quote = code[i];
-        while (++i < end && code[i] !== quote) {
-          if (code[i] === "\\") i++;
-        }
-        break;
-      }
-      case "/":
-        if (code[i + 1] === "/") {
-          while (++i < end && code[i] !== "\n");
-        } else if (code[i + 1] === "*") {
-          while (++i < end && !(code[i] === "*" && code[i + 1] === "/"));
-          i++;
-        }
-        break;
-    }
-  }
-
-  return -1;
-}
-
 function templateElement(value, tail) {
   return t.templateElement({
     tail,
diff --git a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-dom.debug.txt b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-dom.debug.txt
deleted file mode 100644
index 79861b7364d..00000000000
--- a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-dom.debug.txt
+++ /dev/null
@@ -1,5 +0,0 @@
-
-    at packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/template.marko:1:18
-    > 1 | <div class="foo" <!-- the label --> id="bar">Hello</div>
-        |                  ^^^^ HTML comments are not supported in a tag's attributes. Use a JavaScript comment (`// …` or `/* … */`), or move the `<!-- … -->` outside the tag.
-      2 |
\ No newline at end of file
diff --git a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-dom.txt b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-dom.txt
deleted file mode 100644
index 79861b7364d..00000000000
--- a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-dom.txt
+++ /dev/null
@@ -1,5 +0,0 @@
-
-    at packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/template.marko:1:18
-    > 1 | <div class="foo" <!-- the label --> id="bar">Hello</div>
-        |                  ^^^^ HTML comments are not supported in a tag's attributes. Use a JavaScript comment (`// …` or `/* … */`), or move the `<!-- … -->` outside the tag.
-      2 |
\ No newline at end of file
diff --git a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-html.debug.txt b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-html.debug.txt
deleted file mode 100644
index 79861b7364d..00000000000
--- a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-html.debug.txt
+++ /dev/null
@@ -1,5 +0,0 @@
-
-    at packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/template.marko:1:18
-    > 1 | <div class="foo" <!-- the label --> id="bar">Hello</div>
-        |                  ^^^^ HTML comments are not supported in a tag's attributes. Use a JavaScript comment (`// …` or `/* … */`), or move the `<!-- … -->` outside the tag.
-      2 |
\ No newline at end of file
diff --git a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-html.txt b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-html.txt
deleted file mode 100644
index 79861b7364d..00000000000
--- a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/__snapshots__/error-compile-html.txt
+++ /dev/null
@@ -1,5 +0,0 @@
-
-    at packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/template.marko:1:18
-    > 1 | <div class="foo" <!-- the label --> id="bar">Hello</div>
-        |                  ^^^^ HTML comments are not supported in a tag's attributes. Use a JavaScript comment (`// …` or `/* … */`), or move the `<!-- … -->` outside the tag.
-      2 |
\ No newline at end of file
diff --git a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/template.marko b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/template.marko
deleted file mode 100644
index d7a4575fb59..00000000000
--- a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/template.marko
+++ /dev/null
@@ -1 +0,0 @@
-<div class="foo" <!-- the label --> id="bar">Hello</div>
diff --git a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/test.ts b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/test.ts
deleted file mode 100644
index 87d511a1f39..00000000000
--- a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-after-attr/test.ts
+++ /dev/null
@@ -1,5 +0,0 @@
-import type { TestConfig } from "../../main.test";
-
-export const config: TestConfig = {
-  error_compiler: true,
-};
diff --git a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-dom.debug.txt b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-dom.debug.txt
deleted file mode 100644
index 31fd71aa7e5..00000000000
--- a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-dom.debug.txt
+++ /dev/null
@@ -1,8 +0,0 @@
-
-    at packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/template.marko:2:3
-      1 | <div
-    > 2 |   <!-- the label -->
-        |   ^^^^ HTML comments are not supported in a tag's attributes. Use a JavaScript comment (`// …` or `/* … */`), or move the `<!-- … -->` outside the tag.
-      3 |   class="foo"
-      4 | >Hello</div>
-      5 |
\ No newline at end of file
diff --git a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-dom.txt b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-dom.txt
deleted file mode 100644
index 31fd71aa7e5..00000000000
--- a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-dom.txt
+++ /dev/null
@@ -1,8 +0,0 @@
-
-    at packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/template.marko:2:3
-      1 | <div
-    > 2 |   <!-- the label -->
-        |   ^^^^ HTML comments are not supported in a tag's attributes. Use a JavaScript comment (`// …` or `/* … */`), or move the `<!-- … -->` outside the tag.
-      3 |   class="foo"
-      4 | >Hello</div>
-      5 |
\ No newline at end of file
diff --git a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-html.debug.txt b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-html.debug.txt
deleted file mode 100644
index 31fd71aa7e5..00000000000
--- a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-html.debug.txt
+++ /dev/null
@@ -1,8 +0,0 @@
-
-    at packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/template.marko:2:3
-      1 | <div
-    > 2 |   <!-- the label -->
-        |   ^^^^ HTML comments are not supported in a tag's attributes. Use a JavaScript comment (`// …` or `/* … */`), or move the `<!-- … -->` outside the tag.
-      3 |   class="foo"
-      4 | >Hello</div>
-      5 |
\ No newline at end of file
diff --git a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-html.txt b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-html.txt
deleted file mode 100644
index 31fd71aa7e5..00000000000
--- a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/__snapshots__/error-compile-html.txt
+++ /dev/null
@@ -1,8 +0,0 @@
-
-    at packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/template.marko:2:3
-      1 | <div
-    > 2 |   <!-- the label -->
-        |   ^^^^ HTML comments are not supported in a tag's attributes. Use a JavaScript comment (`// …` or `/* … */`), or move the `<!-- … -->` outside the tag.
-      3 |   class="foo"
-      4 | >Hello</div>
-      5 |
\ No newline at end of file
diff --git a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/template.marko b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/template.marko
deleted file mode 100644
index af43a205c7f..00000000000
--- a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/template.marko
+++ /dev/null
@@ -1,4 +0,0 @@
-<div
-  <!-- the label -->
-  class="foo"
->Hello</div>
diff --git a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/test.ts b/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/test.ts
deleted file mode 100644
index 87d511a1f39..00000000000
--- a/packages/runtime-tags/src/__tests__/fixtures/error-html-comment-in-attrs/test.ts
+++ /dev/null
@@ -1,5 +0,0 @@
-import type { TestConfig } from "../../main.test";
-
-export const config: TestConfig = {
-  error_compiler: true,
-};