diff --git a/SKILL.md b/SKILL.md
index 37d3073d..a3c3a2b5 100644
--- a/SKILL.md
+++ b/SKILL.md
@@ -108,6 +108,8 @@ When modifying existing presentations, make **minimal changes** — only touch w
 
 **If any rule fails after editing, fix it before saving.**
 
+Then run the HTML tag-balance check: `python scripts/validate-html.py <file>`. It exits non-zero and names the offending tag on any unclosed or mismatched tag, which catches a broken edit the spec rules do not. Fix any reported tag before saving.
+
 ---
 
 ## Phase 1: Choose Mode
@@ -446,7 +448,7 @@ Always embed inline speaker notes in each slide. If the source had speaker notes
 
 ### Step 5.5: Validate & Save
 
-Before saving, verify all 8 spec rules pass. Fix any that fail. Save the HTML file.
+Before saving, verify all 8 spec rules pass. Fix any that fail. Then run `python scripts/validate-html.py <file>` to confirm every tag is balanced, and fix any unclosed or mismatched tag it names. Save the HTML file.
 
 ---
 
@@ -479,12 +481,13 @@ Deploys the presentation to a permanent shareable URL. Works on any device.
 2. **Check login** — Run `npx vercel whoami`. If not logged in, guide through:
    - Sign up at https://vercel.com/signup (free)
    - Run `vercel login` to authorize
-3. **Deploy** — Run:
+3. **Check confidentiality** - A public deploy ships the whole file, including every inline `<script class="slide-notes">` block, so speaker notes are readable by anyone with the URL. Grep the file for client or project identifiers across both visible text and the `slide-notes` blocks. If the notes must not ship, deploy a stripped copy instead: remove every `<script class="slide-notes">` block from the copy, deploy that copy, and keep the original so the presenter app still has the notes.
+4. **Deploy** — Run:
    ```bash
    bash scripts/deploy.sh <path-to-presentation>
    ```
    Accepts a single HTML file or a folder with index.html.
-4. **Share the URL** — Tell the user the live URL, that it works on any device, and that they can delete it from https://vercel.com/dashboard later.
+5. **Share the URL** — Tell the user the live URL, that it works on any device, and that they can delete it from https://vercel.com/dashboard later.
 
 **Requires:** Node.js + Vercel account (free tier)
 
@@ -523,6 +526,7 @@ Captures each slide as a screenshot and combines into a single PDF. Animations a
 | [libraries.md](references/libraries.md) | CDN libraries: Mermaid.js (diagrams), anime.js (animations), Chart.js (charts) | Phase 3 (when content needs them) |
 | [presentation-layer.md](references/presentation-layer.md) | Shared spec: slide structure, speaker notes, 8 validation rules, navigation | Phase 3 (reference) |
 | [scripts/extract-pptx.py](scripts/extract-pptx.py) | Python script for PPT content extraction | Phase 4 (PPT conversion) |
+| [scripts/validate-html.py](scripts/validate-html.py) | Stdlib HTML tag-balance check, exits non-zero on any unclosed or mismatched tag | Step 5.5 and after any modification |
 | [conversion-patterns.md](references/conversion-patterns.md) | Framework detection patterns and extraction rules | Phase 5 (HTML conversion) |
 | [scripts/deploy.sh](scripts/deploy.sh) | Deploy slides to Vercel for instant sharing | Phase 7 (sharing) |
 | [scripts/export-pdf.sh](scripts/export-pdf.sh) | Export slides to PDF | Phase 7 (sharing) |
diff --git a/references/presentation-layer.md b/references/presentation-layer.md
index b74a593f..4a7d7c6c 100644
--- a/references/presentation-layer.md
+++ b/references/presentation-layer.md
@@ -56,9 +56,11 @@ Every slide **must** include an inline `<script type="application/json" class="s
 
 **Never include:** delivery instructions, transition cues, presentation advice, or meta commentary.
 
-## Spec Rules (8 total)
+**Warning:** these notes ship inside the public artifact. A deployed file carries every `slide-notes` block in plain text, readable by anyone with the URL, unless you strip them before deploy (see SKILL.md Phase 7A).
 
-After generating or modifying ANY presentation, verify all 8 rules:
+## Spec Rules (9 total)
+
+After generating or modifying ANY presentation, verify all 9 rules:
 
 1. `<div class="deck" id="deck">` exists
 2. All slides are `<div class="slide">` with sequential `data-slide="0"` through `data-slide="N"`
@@ -68,6 +70,7 @@ After generating or modifying ANY presentation, verify all 8 rules:
 6. All JS inline (except CDN libraries: Chart.js, Mermaid, anime.js)
 7. No broken numbering gaps after insertions or deletions
 8. `<meta name="generator" content="html-slides vX.Y.Z">` exists in `<head>`
+9. Every tag is balanced. Run `python scripts/validate-html.py <file>`, which exits non-zero and names the offending tag on any unclosed or mismatched tag.
 
 ## Content Density Limits
 
diff --git a/scripts/validate-html.py b/scripts/validate-html.py
new file mode 100644
index 00000000..f26695ea
--- /dev/null
+++ b/scripts/validate-html.py
@@ -0,0 +1,102 @@
+#!/usr/bin/env python3
+"""
+Validate that an HTML slide file has balanced tags.
+Exits non-zero and names the offending tag on any unclosed or mismatched tag.
+
+Usage:
+    python scripts/validate-html.py <input.html>
+"""
+
+import sys
+from html.parser import HTMLParser
+
+# Void elements never carry a closing tag, so they must not push onto the stack.
+VOID_ELEMENTS = frozenset(
+    [
+        "area",
+        "base",
+        "br",
+        "col",
+        "embed",
+        "hr",
+        "img",
+        "input",
+        "link",
+        "meta",
+        "param",
+        "source",
+        "track",
+        "wbr",
+    ]
+)
+
+
+class TagBalanceParser(HTMLParser):
+    """Track a stack of opened non-void tags and record the first imbalance."""
+
+    def __init__(self):
+        super().__init__(convert_charrefs=True)
+        self.stack = []
+        self.error = None
+
+    def handle_starttag(self, tag, attrs):
+        if self.error is not None:
+            return
+        if tag not in VOID_ELEMENTS:
+            self.stack.append((tag, self.getpos()))
+
+    def handle_startendtag(self, tag, attrs):
+        # Self-closing syntax (<tag />) opens and closes in one token, so skip it.
+        return
+
+    def handle_endtag(self, tag):
+        if self.error is not None:
+            return
+        if tag in VOID_ELEMENTS:
+            return
+        if not self.stack:
+            line, _ = self.getpos()
+            self.error = f"unexpected closing tag </{tag}> at line {line} with no open tag"
+            return
+        open_tag, (open_line, _) = self.stack[-1]
+        if open_tag != tag:
+            line, _ = self.getpos()
+            self.error = (
+                f"mismatched closing tag </{tag}> at line {line}, "
+                f"expected </{open_tag}> opened at line {open_line}"
+            )
+            return
+        self.stack.pop()
+
+
+def validate(file_path):
+    """Return an error message string, or None when every tag balances."""
+    with open(file_path, "r", encoding="utf-8") as f:
+        html = f.read()
+
+    parser = TagBalanceParser()
+    parser.feed(html)
+    parser.close()
+
+    if parser.error is not None:
+        return parser.error
+    if parser.stack:
+        tag, (line, _) = parser.stack[0]
+        return f"unclosed tag <{tag}> opened at line {line}"
+    return None
+
+
+if __name__ == "__main__":
+    if len(sys.argv) < 2:
+        print("Usage: python scripts/validate-html.py <input.html>")
+        sys.exit(1)
+
+    input_file = sys.argv[1]
+    error = validate(input_file)
+
+    if error is not None:
+        print(f"FAIL: {input_file}: {error}")
+        sys.exit(1)
+
+    print(f"OK: {input_file}: all tags balanced")
+    sys.exit(0)
diff --git a/scripts/validate-html.test.py b/scripts/validate-html.test.py
new file mode 100644
index 00000000..f5246f0e
--- /dev/null
+++ b/scripts/validate-html.test.py
@@ -0,0 +1,113 @@
+#!/usr/bin/env python3
+"""
+Tests for validate-html.py.
+Run with: python scripts/validate-html.test.py
+
+Stdlib unittest only, no install step, mirroring the script it covers.
+"""
+
+import importlib.util
+import os
+import tempfile
+import unittest
+
+_HERE = os.path.dirname(os.path.abspath(__file__))
+_SPEC = importlib.util.spec_from_file_location(
+    "validate_html", os.path.join(_HERE, "validate-html.py")
+)
+validate_html = importlib.util.module_from_spec(_SPEC)
+_SPEC.loader.exec_module(validate_html)
+
+
+def write_temp(html):
+    """Write HTML to a temp file and return its path for validation."""
+    fd, path = tempfile.mkstemp(suffix=".html")
+    with os.fdopen(fd, "w", encoding="utf-8") as f:
+        f.write(html)
+    return path
+
+
+WELL_FORMED_DECK = """<!DOCTYPE html>
+<html lang="en">
+<head>
+<meta charset="UTF-8">
+<title>Example Deck</title>
+</head>
+<body>
+<div class="deck" id="deck">
+  <div class="slide active" data-slide="0"><h1>Slide One</h1></div>
+  <div class="slide" data-slide="1"><h1>Slide Two</h1></div>
+</div>
+</body>
+</html>
+"""
+
+UNCLOSED_DIV = """<!DOCTYPE html>
+<html>
+<body>
+<div class="deck">
+  <div class="slide">Example bullet
+</div>
+</body>
+</html>
+"""
+
+MISMATCHED_CLOSE = """<!DOCTYPE html>
+<html>
+<body>
+<div class="deck"><span>Section Title</div></span>
+</body>
+</html>
+"""
+
+VOID_ELEMENTS_DECK = """<!DOCTYPE html>
+<html>
+<head><meta charset="UTF-8"><link rel="icon" href="x"></head>
+<body>
+<div class="slide">
+  <img src="a.png">
+  <br>
+  <hr>
+  <input type="text">
+</div>
+</body>
+</html>
+"""
+
+
+class ValidateHtmlTest(unittest.TestCase):
+    def test_well_formed_deck_passes(self):
+        path = write_temp(WELL_FORMED_DECK)
+        try:
+            self.assertIsNone(validate_html.validate(path))
+        finally:
+            os.remove(path)
+
+    def test_unclosed_div_fails_and_names_tag(self):
+        path = write_temp(UNCLOSED_DIV)
+        try:
+            error = validate_html.validate(path)
+            self.assertIsNotNone(error)
+            self.assertIn("div", error)
+        finally:
+            os.remove(path)
+
+    def test_mismatched_close_is_reported(self):
+        path = write_temp(MISMATCHED_CLOSE)
+        try:
+            error = validate_html.validate(path)
+            self.assertIsNotNone(error)
+            self.assertIn("mismatched", error)
+        finally:
+            os.remove(path)
+
+    def test_void_elements_do_not_count_as_unclosed(self):
+        path = write_temp(VOID_ELEMENTS_DECK)
+        try:
+            self.assertIsNone(validate_html.validate(path))
+        finally:
+            os.remove(path)
+
+
+if __name__ == "__main__":
+    unittest.main()