diff --git a/.github/workflows/content-lint.yml b/.github/workflows/content-lint.yml
new file mode 100644
index 0000000..f77c1d6
--- /dev/null
+++ b/.github/workflows/content-lint.yml
@@ -0,0 +1,29 @@
+# Content lint: gate pull requests on textbook authoring conventions.
+#
+# Runs lightweight checks on the Markdown content. Separate from the deploy
+# workflow (jekyll.yml) on purpose: a content-convention miss should block a
+# MERGE, but never take down the live site. This is also the home for future
+# content gates (e.g. the #99 code-block standardization lint).
+name: Content lint
+
+on:
+  pull_request:
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  seo-frontmatter:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.x"
+
+      - name: Check per-page SEO front matter (description:)
+        run: python scripts/check_seo_frontmatter.py
diff --git a/scripts/README.md b/scripts/README.md
index c2d12c2..12e2b79 100644
--- a/scripts/README.md
+++ b/scripts/README.md
@@ -22,6 +22,7 @@ Python utilities that bulk-edit or generate content across the textbook. They ar
 
 | Script | Purpose | Apply flag |
 |---|---|---|
+| [`check_seo_frontmatter.py`](check_seo_frontmatter.py) | **CI gate** — fail if any published page is missing `description:` front matter (reminds about MP4-hero pages lacking a poster). Read-only. | _(none)_ |
 | [`generate_og_posters.py`](generate_og_posters.py) | Generate static OG/social-card poster images from a page's hero `<video>` (MP4) via ffmpeg, and set `image:` front matter. | `--run` |
 | [`fix_embedded_media.py`](fix_embedded_media.py) | Normalize `<video>` inline styles and wrap bare YouTube iframes responsively. | `--run` |
 | [`update_lesson_nav.py`](update_lesson_nav.py) | Migrate old `.btn` lesson nav to card-style `<nav class="lesson-nav">` (rewrites `.md`→`.html`). | `--run` |
@@ -29,6 +30,19 @@ Python utilities that bulk-edit or generate content across the textbook. They ar
 
 ## Details
 
+### `check_seo_frontmatter.py`
+
+Enforces the per-page SEO convention: every published page must set `description:`.
+Run by the **Content lint** GitHub Actions workflow (`.github/workflows/content-lint.yml`)
+on every pull request — it exits non-zero (failing the PR check, but never the deploy) if
+any page is missing it. Pages marked `nav_exclude: true`/`search_exclude: true`, plus the
+contributor docs and deprecated pages, are exempt. `image:` is advisory: the script only
+prints a reminder when an MP4-hero page has no poster yet. Read-only; takes no flags.
+
+```bash
+python scripts/check_seo_frontmatter.py    # exit 0 = all good, 1 = a page is missing description:
+```
+
 ### `generate_og_posters.py`
 
 For pages whose **first/hero media is an MP4 `<video>`**, extracts a representative
diff --git a/scripts/check_seo_frontmatter.py b/scripts/check_seo_frontmatter.py
new file mode 100644
index 0000000..84f3e62
--- /dev/null
+++ b/scripts/check_seo_frontmatter.py
@@ -0,0 +1,120 @@
+"""
+check_seo_frontmatter.py — CI gate: every published page must set `description:`.
+
+Per-page `description:` (and, where possible, `image:`) drives search snippets and
+social link-preview cards via jekyll-seo-tag. This script enforces the convention so
+new content (and in-flight branches) can't quietly regress to the generic site card.
+See the "SEO and social cards" section of website-dev.md.
+
+Rules:
+  - A "page" = a .md file with YAML front matter containing `layout:`.
+  - Every page MUST have a non-empty `description:` — EXCEPT:
+      * pages marked draft via `nav_exclude: true` or `search_exclude: true`, and
+      * paths in IGNORE (contributor docs, deprecated pages).
+    A draft becomes subject to the rule as soon as it's published (nav_exclude removed).
+  - `image:` is ADVISORY: a page whose hero is an MP4 <video> but has no `image:`
+    yet gets a non-fatal reminder to run scripts/generate_og_posters.py. Pages may
+    legitimately have no image (they fall back to the site card).
+
+Exit code: 1 if any required page is missing `description:` (fails the CI check);
+0 otherwise. Advisory image reminders never affect the exit code.
+
+Usage:
+    python scripts/check_seo_frontmatter.py
+"""
+
+import re
+import sys
+from pathlib import Path
+
+DOCS_DIR = "."
+SKIP_DIRS = {"_site", ".git", "node_modules", "vendor", ".jekyll-cache",
+             "scripts", "_includes", "_layouts", "_data", "_sass", "assets"}
+
+# Pages exempt from the description: requirement (contributor docs, deprecated).
+IGNORE = {
+    "website-dev.md", "website-install.md", "teaching-notes.md",
+    "website-content-ideas.md", "README.md", "LICENSE.md", "CLAUDE.md",
+    "404.md", "arduino/potentiometers-old.md",
+}
+
+FRONT_MATTER_RE = re.compile(r"^---\s*\n(.*?)\n---\s*\n", re.DOTALL)
+HTML_COMMENT_RE = re.compile(r"<!--.*?-->", re.DOTALL)
+MEDIA_TOKEN_RE = re.compile(r"<video\b|!\[|<img\b|<iframe\b", re.IGNORECASE)
+MP4_SOURCE_RE = re.compile(r'<source\b[^>]*\.mp4"', re.IGNORECASE)
+
+
+def front_matter(content):
+    m = FRONT_MATTER_RE.match(content)
+    return (m.group(1), content[m.end():]) if m else (None, content)
+
+
+def fm_has(fm, key):
+    """True if front matter has a non-empty value for `key`."""
+    m = re.search(rf"^{key}:\s*(.+?)\s*$", fm, re.MULTILINE)
+    return bool(m and m.group(1).strip() not in ("", '""', "''"))
+
+
+def fm_true(fm, key):
+    return bool(re.search(rf"^{key}:\s*true\s*$", fm, re.MULTILINE | re.IGNORECASE))
+
+
+def hero_is_mp4(body):
+    visible = HTML_COMMENT_RE.sub("", body)
+    first = MEDIA_TOKEN_RE.search(visible)
+    if not first or not visible[first.start():first.end()].lower().startswith("<video"):
+        return False
+    return bool(MP4_SOURCE_RE.search(visible, first.start()))
+
+
+def rel(p):
+    return str(p).replace("\\", "/").lstrip("./")
+
+
+def main():
+    missing_desc = []
+    image_reminders = []
+    checked = 0
+
+    for path in sorted(Path(DOCS_DIR).rglob("*.md")):
+        if any(part in SKIP_DIRS for part in path.parts):
+            continue
+        relpath = rel(path)
+        if relpath in IGNORE:
+            continue
+
+        fm, body = front_matter(path.read_text(encoding="utf-8"))
+        if fm is None or not re.search(r"^layout:", fm, re.MULTILINE):
+            continue  # not a page
+        if fm_true(fm, "nav_exclude") or fm_true(fm, "search_exclude"):
+            continue  # draft / hidden
+
+        checked += 1
+        if not fm_has(fm, "description"):
+            missing_desc.append(relpath)
+        if not fm_has(fm, "image") and hero_is_mp4(body):
+            image_reminders.append(relpath)
+
+    print(f"Checked {checked} published page(s).")
+
+    if image_reminders:
+        print(f"\nReminder ({len(image_reminders)}): MP4-hero page(s) with no `image:` "
+              f"— run `python scripts/generate_og_posters.py --run`:")
+        for p in image_reminders:
+            print(f"  - {p}")
+
+    if missing_desc:
+        print(f"\nERROR: {len(missing_desc)} published page(s) missing `description:` "
+              f"front matter:")
+        for p in missing_desc:
+            print(f"  - {p}")
+        print("\nAdd a `description:` (see website-dev.md -> 'SEO and social cards'). "
+              "Drafts can set `nav_exclude: true` to defer.")
+        return 1
+
+    print("\nAll published pages have `description:`. OK")
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())
diff --git a/website-dev.md b/website-dev.md
index 8acb5b3..d0eae0d 100644
--- a/website-dev.md
+++ b/website-dev.md
@@ -111,6 +111,34 @@ To verify after a build, grep the output, e.g.:
 grep -oiE '<meta (name|property)="(og:image|og:description|description)" content="[^"]*"' _site/arduino/led-fade.html
 ```
 
+### New pages and enforcement
+
+This is **required**, not optional. A CI check (`scripts/check_seo_frontmatter.py`, run by
+the **Content lint** workflow on every pull request) fails the PR if any published page is
+missing `description:`. So when you author a new lesson, start from this minimal front matter:
+
+```yaml
+---
+layout: default
+title: "Your Lesson Title"
+description: "One or two sentences (≤160 chars) on what the reader learns or builds."
+# image:  ← add per the rules above; for an MP4 hero, run the poster script (below) instead
+parent: Your Section
+nav_order: 1
+---
+```
+
+If a page isn't ready to publish, mark it `nav_exclude: true` (or `search_exclude: true`) and
+the check skips it until you publish it. The `image:` key is advisory — the check only *reminds*
+you when an MP4-hero page has no poster yet.
+
+For a new page whose hero is an **MP4 `<video>`**, generate its social poster (and have `image:`
+set for you) with:
+
+```bash
+python scripts/generate_og_posters.py --run <module>/<your-page>.md
+```
+
 ## Code highlighting
 <!-- Code snippet highlighting: https://jekyllrb.com/docs/liquid/tags/#code-snippet-highlighting -->