diff --git a/.github/workflows/pages.yml b/.github/workflows/pages.yml
index 79e7f6f..1c4d465 100644
--- a/.github/workflows/pages.yml
+++ b/.github/workflows/pages.yml
@@ -52,6 +52,29 @@ jobs:
             echo "No HTML files found yet (layouts not implemented). Skipping Pagefind."
           fi
 
+      - name: Assert CSS bundle budget (≤ 4200 B gz)
+        run: |
+          css=$(find exampleSite/public -name 'tsuki.bundle.*.css' -print -quit)
+          if [ -z "$css" ]; then
+            echo "::error::Could not locate tsuki.bundle.*.css under exampleSite/public"
+            exit 1
+          fi
+          sz=$(gzip -9 -c "$css" | wc -c)
+          echo "tsuki bundle gz: ${sz} B (limit 4200)"
+          if [ "$sz" -gt 4200 ]; then
+            echo "::error::CSS bundle exceeds 4200 B gz budget (was ${sz} B)"
+            exit 1
+          fi
+
+      - name: Smoke tests (SEO + a11y + features regression guard)
+        run: ./scripts/smoke-tests.sh exampleSite/public
+
+      - name: htmltest (broken internal links + HTML5 validation)
+        # TODO before v0.2.0 tag: pin @master to a commit SHA (supply-chain hygiene).
+        uses: wjdp/htmltest-action@master
+        with:
+          config: .htmltest.yml
+
       - name: Upload artifact
         uses: actions/upload-pages-artifact@v5
         with:
diff --git a/.htmltest.yml b/.htmltest.yml
new file mode 100644
index 0000000..34194ef
--- /dev/null
+++ b/.htmltest.yml
@@ -0,0 +1,10 @@
+DirectoryPath: exampleSite/public
+CheckExternal: false
+IgnoreDirectoryMissingTrailingSlash: true
+IgnoreInternalEmptyHash: true
+IgnoreEmptyHref: false
+EnforceHTML5: true
+IgnoreURLs:
+  - "^https://giscus.app"
+  - "^https://github.com/"
+  - "^https://gohugo.io"
diff --git a/scripts/smoke-tests.sh b/scripts/smoke-tests.sh
new file mode 100755
index 0000000..bb6049a
--- /dev/null
+++ b/scripts/smoke-tests.sh
@@ -0,0 +1,81 @@
+#!/usr/bin/env bash
+# tsuki theme post-build smoke tests
+# Run after hugo build of exampleSite to assert key features still emit.
+# Usage: scripts/smoke-tests.sh [<public-dir>]
+# Default <public-dir>: exampleSite/public
+
+set -euo pipefail
+
+public_dir="${1:-exampleSite/public}"
+fail=0
+
+assert() {
+  local label="$1" pattern="$2" file="$3" min="${4:-1}"
+  local count
+  count=$(grep -cE "$pattern" "$file" 2>/dev/null || true)
+  if [ "$count" -lt "$min" ]; then
+    echo "::error file=$file::FAIL [$label] expected ≥${min} matches of /$pattern/, got ${count}"
+    fail=1
+  else
+    echo "  ok   [$label] ${count} match(es) in ${file##$public_dir/}"
+  fi
+}
+
+[ -d "$public_dir" ] || { echo "::error::Public dir not found: $public_dir"; exit 2; }
+
+# Pick a known leaf post (any 2026 post will do).
+post=$(find "$public_dir" -name 'index.html' -path '*/2026/*' -not -path '*/page/*' | head -1 || true)
+[ -n "$post" ] || { echo "::error::No demo post HTML found under $public_dir"; exit 2; }
+home="$public_dir/index.html"
+[ -f "$home" ] || { echo "::error::No home HTML at $home"; exit 2; }
+
+echo "Smoke testing tsuki build:"
+echo "  home: $home"
+echo "  post: $post"
+echo
+
+# SEO (Phase 3)
+assert "JSON-LD on post"     '<script type=application/ld\+json>'           "$post"
+assert "no JSON-LD on home"  '<script type=application/ld\+json>'           "$home" 0
+assert "OG image emits"      '<meta property="og:image"'                    "$post"
+assert "Twitter image emits" '<meta name=twitter:image'                     "$post"
+assert "OG locale emits"     '<meta property="og:locale"'                   "$post"
+
+# Accessibility (Phase 2)
+assert "skip-link present"   'class=skip-link'                              "$home"
+assert "main id anchor"      '<main id=main>'                               "$home"
+
+# Render-link hook (Phase 2): scan all post HTML for at least one external rel marker.
+# Pipeline guarded — `grep -r` exits 1 on zero matches; `set -o pipefail` would kill the script.
+rel_count=$({ grep -roE 'rel="noopener noreferrer"' "$public_dir" --include='index.html' || true; } | wc -l)
+if [ "$rel_count" -lt 1 ]; then
+  echo "::error::FAIL [render-link rel] expected ≥1 'rel=\"noopener noreferrer\"' across built HTML, got 0 — demo posts may have lost their external links"
+  fail=1
+else
+  echo "  ok   [render-link rel] ${rel_count} match(es) across $public_dir"
+fi
+
+# Author UX (Phase 4)
+assert "reading time byline" 'reading-time'                                 "$post"
+
+# Discovery (Phase 5)
+assert "related-posts aside" 'class=related-posts'                          "$post"
+
+# CSS budget (Phase 1) — duplicates the workflow assert; included so local runs catch it too.
+css=$(find "$public_dir" -name 'tsuki.bundle.*.css' -print -quit || true)
+if [ -n "$css" ]; then
+  sz=$(gzip -9 -c "$css" | wc -c)
+  if [ "$sz" -gt 4200 ]; then
+    echo "::error::CSS bundle gz ${sz} B exceeds 4200 B budget"
+    fail=1
+  else
+    echo "  ok   [CSS budget] ${sz} B / 4200 B"
+  fi
+else
+  echo "::error::Could not locate tsuki.bundle.*.css"
+  fail=1
+fi
+
+echo
+[ "$fail" -eq 0 ] && echo "All smoke tests passed." || echo "Smoke tests failed."
+exit "$fail"