diff --git a/packages/lint/src/context.ts b/packages/lint/src/context.ts
index 7ee0d7516..3e71b29a3 100644
--- a/packages/lint/src/context.ts
+++ b/packages/lint/src/context.ts
@@ -34,8 +34,15 @@ export function buildLintContext(html: string, options: HyperframeLinterOptions
   // hijack the boundary match below. Linear + fixpoint (see stripHtmlComments) to
   // stay ReDoS-free and catch markers that re-form when a comment is removed.
   let source = stripHtmlComments(rawSource);
+  const sourceWithoutTemplates = source.replace(
+    /<template\b[^>]*>[\s\S]*?<\/template(?:\s[^>]*)?>/gi,
+    " ",
+  );
   const templateMatch = source.match(/<template[^>]*>([\s\S]*)<\/template>/i);
-  if (templateMatch?.[1]) source = templateMatch[1];
+  // Some sub-composition files are HTML shells whose real root lives inside a
+  // <template>. Keep nested templates intact when the visible document already
+  // has a composition root; only unwrap when no root exists outside templates.
+  if (templateMatch?.[1] && !findRootTag(sourceWithoutTemplates)) source = templateMatch[1];
 
   const tags = extractOpenTags(source);
   const styles = [
diff --git a/packages/lint/src/rules/core.test.ts b/packages/lint/src/rules/core.test.ts
index e30f8f74b..5f601e858 100644
--- a/packages/lint/src/rules/core.test.ts
+++ b/packages/lint/src/rules/core.test.ts
@@ -370,6 +370,87 @@ body {
     expect(finding).toBeUndefined();
   });
 
+  it("reports error when CSS block comment syntax leaks into visible markup", async () => {
+    const html = compositionWithBodyPrefix(
+      "",
+      `
+    /* Main Content Block */
+    <div class="editorial-block">Hello</div>
+`,
+    );
+    const result = await lintHyperframeHtml(html);
+    const finding = result.findings.find((f) => f.code === "visible_markup_comment");
+
+    expect(finding).toBeDefined();
+    expect(finding?.severity).toBe("error");
+    expect(finding?.message).toContain("visible HTML markup");
+    expect(finding?.snippet).toContain("Main Content Block");
+  });
+
+  it("reports error when a misbalanced style block leaves block comment syntax visible", async () => {
+    const html = compositionWithBodyPrefix(
+      "",
+      `
+    <style>
+      .editorial-block { color: #fff; }
+    </style>
+    </style>
+    /* Main Content Block */
+    <div class="editorial-block">Hello</div>
+`,
+    );
+    const result = await lintHyperframeHtml(html);
+    const finding = result.findings.find((f) => f.code === "visible_markup_comment");
+
+    expect(finding).toBeDefined();
+    expect(finding?.snippet).toContain("Main Content Block");
+  });
+
+  it("does not report block comments inside style or script blocks", async () => {
+    const html = `
+<html>
+<head>
+  <title>/* tab name */ Particle Field</title>
+  <style>
+    /* Layout reset */
+    body { margin: 0; }
+  </style>
+  <noscript>/* fallback note */</noscript>
+</head>
+<body>
+  <div data-composition-id="c1" data-width="1920" data-height="1080"></div>
+  <script>
+    /* Timeline registry */
+    window.__timelines = {};
+  </script>
+</body>
+</html>`;
+    const result = await lintHyperframeHtml(html);
+    const finding = result.findings.find((f) => f.code === "visible_markup_comment");
+
+    expect(finding).toBeUndefined();
+  });
+
+  it("does not report block comments in attributes, html comments, or protected text contexts", async () => {
+    const html = compositionWithBodyPrefix(
+      "",
+      `
+    <!-- /* hidden implementation note */ -->
+    <div data-note="/* attribute note */"></div>
+    <div data-note="a > b /* quoted attribute note */"></div>
+    <pre>/* visible code sample */</pre>
+    <code>/* visible inline code sample */</code>
+    <textarea>/* editable code sample */</textarea>
+    <template>/* template-only note */</template>
+    <svg viewBox="0 0 100 20"><text x="0" y="15">/* svg label */</text></svg>
+`,
+    );
+    const result = await lintHyperframeHtml(html);
+    const finding = result.findings.find((f) => f.code === "visible_markup_comment");
+
+    expect(finding).toBeUndefined();
+  });
+
   it("reports error when a stray style close tag is left in the document head", async () => {
     const html = compositionWithHead(`
   <style>
diff --git a/packages/lint/src/rules/core.ts b/packages/lint/src/rules/core.ts
index 6a7ec77dd..fec78abee 100644
--- a/packages/lint/src/rules/core.ts
+++ b/packages/lint/src/rules/core.ts
@@ -68,6 +68,14 @@ const ORPHAN_CSS_AT_RULE_PATTERN =
   /(?:^|\s)@(?:container|font-face|keyframes|layer|media|page|property|scope|supports)[^{<]*\{[\s\S]*?:[\s\S]*?\}/i;
 const ORPHAN_CSS_RULE_PATTERN =
   /(?:^|\s)(?:\/\*[\s\S]*?\*\/\s*)?(?:@[a-z-]+[^{}<]*|[.#][\w-]+[^{}<]*|[a-z][\w-]*(?:\s+[.#:[\w-][^{}<]*)?)\s*\{[^{}]*:[^{}]*\}/i;
+const VISIBLE_MARKUP_COMMENT_PATTERN = /\/\*[\s\S]*?\*\//g;
+const VISIBLE_MARKUP_COMMENT_PROTECTED_BLOCK_PATTERN =
+  /<(style|script|template|title|noscript|pre|code|textarea|text)\b[^>]*>[\s\S]*?<\/\1(?:\s[^>]*)?>/gi;
+
+interface SourceRange {
+  start: number;
+  end: number;
+}
 
 function findCodeFenceLeak(headWithoutValidBlocks: string): string | null {
   return MARKDOWN_CODE_FENCE_PATTERN.exec(headWithoutValidBlocks)?.[0] ?? null;
@@ -127,6 +135,50 @@ function findLeakedTextBeforeCompositionRoot(
   return findLeakedTextInHeadContent(source.slice(prefixStart, prefixEnd));
 }
 
+function findProtectedVisibleMarkupRanges(source: string): SourceRange[] {
+  const ranges: SourceRange[] = [];
+  for (const match of source.matchAll(VISIBLE_MARKUP_COMMENT_PROTECTED_BLOCK_PATTERN)) {
+    ranges.push({ start: match.index, end: match.index + match[0].length });
+  }
+  return ranges;
+}
+
+function isInsideSourceRange(index: number, ranges: SourceRange[]): boolean {
+  return ranges.some((range) => range.start <= index && index < range.end);
+}
+
+function isInsideHtmlTag(source: string, index: number): boolean {
+  let inTag = false;
+  let quote: '"' | "'" | null = null;
+  for (let i = 0; i < index; i++) {
+    const char = source[i];
+    if (!inTag) {
+      if (char === "<") inTag = true;
+      continue;
+    }
+    if (quote) {
+      if (char === quote) quote = null;
+      continue;
+    }
+    if (char === '"' || char === "'") {
+      quote = char;
+    } else if (char === ">") {
+      inTag = false;
+    }
+  }
+  return inTag;
+}
+
+function findVisibleMarkupCommentLeak(source: string): string | null {
+  const protectedRanges = findProtectedVisibleMarkupRanges(source);
+  for (const match of source.matchAll(VISIBLE_MARKUP_COMMENT_PATTERN)) {
+    if (isInsideHtmlTag(source, match.index)) continue;
+    if (isInsideSourceRange(match.index, protectedRanges)) continue;
+    return match[0];
+  }
+  return null;
+}
+
 export const coreRules: Array<(ctx: LintContext) => HyperframeLintFinding[]> = [
   // root_missing_composition_id + root_missing_dimensions
   ({ rootTag }) => {
@@ -174,6 +226,23 @@ export const coreRules: Array<(ctx: LintContext) => HyperframeLintFinding[]> = [
     ];
   },
 
+  // visible_markup_comment
+  ({ source }) => {
+    const snippet = findVisibleMarkupCommentLeak(source);
+    if (!snippet) return [];
+    return [
+      {
+        code: "visible_markup_comment",
+        severity: "error",
+        message:
+          "CSS/JS block comment syntax (`/* ... */`) appears in visible HTML markup. HTML only treats `<!-- ... -->` as comments, so this renders as on-screen text.",
+        fixHint:
+          "Remove the text or convert it to a real HTML comment (`<!-- ... -->`). Keep CSS comments inside `<style>` and JS comments inside `<script>`.",
+        snippet: truncateSnippet(snippet),
+      },
+    ];
+  },
+
   // missing_timeline_registry + timeline_registry_missing_init
   ({ source, rawSource, options }) => {
     // Sub-compositions inherit window.__timelines from the host composition
