diff --git a/Makefile b/Makefile
index eb2343f..0aca222 100644
--- a/Makefile
+++ b/Makefile
@@ -40,6 +40,9 @@ parity-lit: ## Confirm Lit elements honour the IR contract + render (browser). E
 designbook-refresh: ## Refresh routine: check->pull->sync->ir->adapt-lit->(drift triage)->parity. ARGS=--no-pull etc.
 	$(NODE) scripts/designbook-refresh.mjs $(ARGS)
 
+adapt-plain-css: ## Second-adapter SMOKE: prove a non-Lit adapter consumes the same ir/ (WHYNOT-WP-0002 T10).
+	$(NODE) adapters/plain-css/adapt.mjs
+
 recent-changes: ## Regenerate RecentChanges.md (alias of the reporter; --range supported).
 	$(NODE) scripts/designbook-sync.mjs $(ARGS)
 
diff --git a/adapters/plain-css/README.md b/adapters/plain-css/README.md
new file mode 100644
index 0000000..f982e7a
--- /dev/null
+++ b/adapters/plain-css/README.md
@@ -0,0 +1,31 @@
+# plain-css adapter — second-adapter smoke (WHYNOT-WP-0002 · T10)
+
+> **This is not a finished stack.** It exists to prove the **IR/adapter boundary**:
+> that a second, non-Lit adapter can consume the *same* `ir/` and honour the *same*
+> [`adapters/ADAPTER_CONTRACT.md`](../ADAPTER_CONTRACT.md). The deliverable is
+> confidence in the seam, so the architecture can later be lifted into a
+> coulomb-level tool — **not** a usable plain-CSS component kit.
+
+Run with **`make adapt-plain-css`** (`adapters/plain-css/adapt.mjs`).
+
+## What it proves
+
+| Contract concern | This adapter | Same as Lit? |
+|---|---|---|
+| Input is `ir/` only | reads `ir/tokens.json` + `ir/components/*.json` | ✓ identical inputs |
+| Tokens fully generated | → `adapters/plain-css/tokens.css` (CSS custom properties, deterministic no-op re-run) | ✓ same discipline, different target |
+| New component → stub | write-once class stub `adapters/plain-css/stubs/<tag>.css` (base + variant modifier classes from the contract) | ✓ write-once, into a staging dir |
+| Drift roll-up | `adapters/plain-css/_report.json` in the contract's report shape (`stack`, `generatedAt`, `components[]`) | ✓ portable shape |
+| Exit codes | `0` ok · `2` usage · `3` new/drift · `5` internal | ✓ shared convention |
+
+Because plain-CSS has no hand-authored source, **every** IR component reports
+`status: "new"` and gets a stub — exactly the contract's new-component path. That a
+totally different stack reuses the same IR, the same report shape, and the same exit
+codes — with **zero changes to `ir/`** — is the proof the boundary holds.
+
+## What it deliberately does NOT do
+
+No real CSS appearance, no parity, no full component set, no integration into the
+repo's build. Finishing a plain-CSS (or Vue/Svelte/…) stack is future work; the seam
+is what T10 validates. See `DesignSystemIntroduction.md` §5.1 and the Lit reference
+adapter (`adapters/lit/`) for the full implementation.
diff --git a/adapters/plain-css/_report.json b/adapters/plain-css/_report.json
new file mode 100644
index 0000000..c7ba13a
--- /dev/null
+++ b/adapters/plain-css/_report.json
@@ -0,0 +1,106 @@
+{
+  "stack": "plain-css",
+  "generatedAt": "2026-06-30T07:20:50.544Z",
+  "components": [
+    {
+      "name": "Button",
+      "status": "new",
+      "issues": [
+        {
+          "kind": "no-counterpart",
+          "detail": "no .wn-button class — stub generated."
+        }
+      ]
+    },
+    {
+      "name": "Eyebrow",
+      "status": "new",
+      "issues": [
+        {
+          "kind": "no-counterpart",
+          "detail": "no .wn-eyebrow class — stub generated."
+        }
+      ]
+    },
+    {
+      "name": "Icon",
+      "status": "new",
+      "issues": [
+        {
+          "kind": "no-counterpart",
+          "detail": "no .wn-icon class — stub generated."
+        }
+      ]
+    },
+    {
+      "name": "PageHeader",
+      "status": "new",
+      "issues": [
+        {
+          "kind": "no-counterpart",
+          "detail": "no .wn-page-header class — stub generated."
+        }
+      ]
+    },
+    {
+      "name": "PipelineStrip",
+      "status": "new",
+      "issues": [
+        {
+          "kind": "no-counterpart",
+          "detail": "no .wn-pipeline-strip class — stub generated."
+        }
+      ]
+    },
+    {
+      "name": "Sidebar",
+      "status": "new",
+      "issues": [
+        {
+          "kind": "no-counterpart",
+          "detail": "no .wn-sidebar class — stub generated."
+        }
+      ]
+    },
+    {
+      "name": "StageDot",
+      "status": "new",
+      "issues": [
+        {
+          "kind": "no-counterpart",
+          "detail": "no .wn-stage-dot class — stub generated."
+        }
+      ]
+    },
+    {
+      "name": "Stamp",
+      "status": "new",
+      "issues": [
+        {
+          "kind": "no-counterpart",
+          "detail": "no .wn-stamp class — stub generated."
+        }
+      ]
+    },
+    {
+      "name": "Tag",
+      "status": "new",
+      "issues": [
+        {
+          "kind": "no-counterpart",
+          "detail": "no .wn-tag class — stub generated."
+        }
+      ]
+    },
+    {
+      "name": "TopNav",
+      "status": "new",
+      "issues": [
+        {
+          "kind": "no-counterpart",
+          "detail": "no .wn-top-nav class — stub generated."
+        }
+      ]
+    }
+  ]
+}
diff --git a/adapters/plain-css/adapt.mjs b/adapters/plain-css/adapt.mjs
new file mode 100644
index 0000000..940e07e
--- /dev/null
+++ b/adapters/plain-css/adapt.mjs
@@ -0,0 +1,102 @@
+#!/usr/bin/env node
+// =============================================================
+// adapters/plain-css/adapt.mjs — second-adapter SMOKE (WHYNOT-WP-0002 · T10)
+//
+// NOT a finished stack. This exists only to prove the IR/adapter *boundary*:
+// a non-Lit adapter consuming the very same ir/ and honouring the same
+// adapters/ADAPTER_CONTRACT.md (token full-gen + stub/drift + exit codes).
+// The deliverable is confidence in the seam, not a usable plain-CSS kit.
+//
+// It does two contract things from ir/ and nothing Lit-specific:
+//   • tokens  → fully generated CSS custom properties (deterministic no-op re-run)
+//   • components → write-once class stubs + a drift roll-up in the contract shape
+//     (every component is "new" here — plain-CSS has no existing source to drift).
+//
+// Exit codes (shared contract): 0 ok · 2 usage · 3 drift/new · 5 internal.
+// Run: `make adapt-plain-css`.
+// =============================================================
+import { readFileSync, readdirSync, writeFileSync, existsSync, mkdirSync, rmSync } from "node:fs";
+import { join, dirname } from "node:path";
+import { fileURLToPath } from "node:url";
+
+const REPO = join(dirname(fileURLToPath(import.meta.url)), "..", "..");
+const IR = join(REPO, "ir");
+const HERE = join(REPO, "adapters", "plain-css");
+const STUBS = join(HERE, "stubs");
+
+const refToVar = (v) => {
+  const m = /^\{[A-Za-z0-9]+\.([A-Za-z0-9-]+)\}$/.exec(String(v).trim());
+  return m ? `var(--${m[1]})` : v;
+};
+
+function generateTokens() {
+  const tokens = JSON.parse(readFileSync(join(IR, "tokens.json"), "utf8"));
+  const lines = ["/* @generated by adapters/plain-css (WHYNOT-WP-0002 T10) from ir/tokens.json — DO NOT EDIT. */", ":root {"];
+  let n = 0;
+  for (const [group, entries] of Object.entries(tokens)) {
+    lines.push(`  /* ${group} */`);
+    for (const [key, tok] of Object.entries(entries)) {
+      if (key === "$type") continue;
+      lines.push(`  --${key}: ${refToVar(tok.$value)};`); n++;
+    }
+  }
+  lines.push("}", "");
+  const out = join(HERE, "tokens.css");
+  const body = lines.join("\n");
+  const before = existsSync(out) ? readFileSync(out, "utf8") : null;
+  if (before === body) { console.log(`tokens: up to date (${n} custom properties, no change).`); return; }
+  writeFileSync(out, body);
+  console.log(`tokens: regenerated ${n} custom properties → adapters/plain-css/tokens.css`);
+}
+
+function renderClassStub(c) {
+  const base = c.tag; // reuse the contract tag as the base class name
+  const lines = [
+    `/* @generated STUB by adapters/plain-css (T10) from ir/components/${c.name}.json — write-once. */`,
+    `/* ${c.name}: ${c.description} */`,
+    `.${base} { /* TODO: base appearance per ir/exemplars/${c.name}.html */ }`,
+  ];
+  for (const v of c.variants || []) {
+    for (const val of v.values) lines.push(`.${base}--${val} { /* ${v.axis}=${val} */ }`);
+  }
+  return lines.join("\n") + "\n";
+}
+
+function scaffold() {
+  const contracts = readdirSync(join(IR, "components")).filter((f) => f.endsWith(".json"))
+    .map((f) => JSON.parse(readFileSync(join(IR, "components", f), "utf8")))
+    .sort((a, b) => a.name.localeCompare(b.name));
+
+  // Plain-CSS has no hand-authored source, so every IR component is "new".
+  const components = contracts.map((c) => ({ name: c.name, status: "new",
+    issues: [{ kind: "no-counterpart", detail: `no .${c.tag} class — stub generated.` }] }));
+
+  mkdirSync(STUBS, { recursive: true });
+  let stubbed = 0;
+  for (const c of contracts) {
+    const out = join(STUBS, `${c.tag}.css`);
+    if (!existsSync(out)) { writeFileSync(out, renderClassStub(c)); stubbed++; }
+  }
+  // Drift roll-up — same shape as the Lit adapter's, proving the contract is portable.
+  const reportPath = join(HERE, "_report.json");
+  let generatedAt = new Date().toISOString();
+  if (existsSync(reportPath)) {
+    try { const prev = JSON.parse(readFileSync(reportPath, "utf8"));
+      if (JSON.stringify(prev.components) === JSON.stringify(components) && prev.generatedAt) generatedAt = prev.generatedAt;
+    } catch { /* fresh */ }
+  }
+  writeFileSync(reportPath, JSON.stringify({ stack: "plain-css", generatedAt, components }, null, 2) + "\n");
+  console.log(`scaffold: ${components.length} components, all new (${stubbed} stub${stubbed === 1 ? "" : "s"} written) → adapters/plain-css/stubs/`);
+  return components.length > 0;
+}
+
+function main() {
+  if (!existsSync(join(IR, "tokens.json"))) { console.error("No ir/ — run `make ir` first."); process.exit(2); }
+  generateTokens();
+  const isNew = scaffold();
+  console.log("\nadapt-plain-css: SMOKE only — proves a non-Lit adapter consumes the same ir/ and");
+  console.log("emits the same contract shapes. Not a finished stack (see adapters/plain-css/README.md).");
+  if (isNew) process.exit(3); // new components present (contract: 3) — expected for a fresh stack
+}
+
+main();
diff --git a/adapters/plain-css/stubs/wn-button.css b/adapters/plain-css/stubs/wn-button.css
new file mode 100644
index 0000000..9c6f492
--- /dev/null
+++ b/adapters/plain-css/stubs/wn-button.css
@@ -0,0 +1,6 @@
+/* @generated STUB by adapters/plain-css (T10) from ir/components/Button.json — write-once. */
+/* Button: Button — extracted from designbook ui_kits/whynot-control/Atoms.jsx. */
+.wn-button { /* TODO: base appearance per ir/exemplars/Button.html */ }
+.wn-button--secondary { /* variant=secondary */ }
+.wn-button--primary { /* variant=primary */ }
+.wn-button--ghost { /* variant=ghost */ }
diff --git a/adapters/plain-css/stubs/wn-eyebrow.css b/adapters/plain-css/stubs/wn-eyebrow.css
new file mode 100644
index 0000000..f9a13ed
--- /dev/null
+++ b/adapters/plain-css/stubs/wn-eyebrow.css
@@ -0,0 +1,3 @@
+/* @generated STUB by adapters/plain-css (T10) from ir/components/Eyebrow.json — write-once. */
+/* Eyebrow: Eyebrow — extracted from designbook ui_kits/whynot-control/Atoms.jsx. */
+.wn-eyebrow { /* TODO: base appearance per ir/exemplars/Eyebrow.html */ }
diff --git a/adapters/plain-css/stubs/wn-icon.css b/adapters/plain-css/stubs/wn-icon.css
new file mode 100644
index 0000000..13a448e
--- /dev/null
+++ b/adapters/plain-css/stubs/wn-icon.css
@@ -0,0 +1,3 @@
+/* @generated STUB by adapters/plain-css (T10) from ir/components/Icon.json — write-once. */
+/* Icon: Icon — extracted from designbook ui_kits/whynot-control/Atoms.jsx. */
+.wn-icon { /* TODO: base appearance per ir/exemplars/Icon.html */ }
diff --git a/adapters/plain-css/stubs/wn-page-header.css b/adapters/plain-css/stubs/wn-page-header.css
new file mode 100644
index 0000000..15c8c07
--- /dev/null
+++ b/adapters/plain-css/stubs/wn-page-header.css
@@ -0,0 +1,3 @@
+/* @generated STUB by adapters/plain-css (T10) from ir/components/PageHeader.json — write-once. */
+/* PageHeader: PageHeader — extracted from designbook ui_kits/whynot-control/Chrome.jsx. */
+.wn-page-header { /* TODO: base appearance per ir/exemplars/PageHeader.html */ }
diff --git a/adapters/plain-css/stubs/wn-pipeline-strip.css b/adapters/plain-css/stubs/wn-pipeline-strip.css
new file mode 100644
index 0000000..e187451
--- /dev/null
+++ b/adapters/plain-css/stubs/wn-pipeline-strip.css
@@ -0,0 +1,3 @@
+/* @generated STUB by adapters/plain-css (T10) from ir/components/PipelineStrip.json — write-once. */
+/* PipelineStrip: PipelineStrip — extracted from designbook ui_kits/whynot-control/Chrome.jsx. */
+.wn-pipeline-strip { /* TODO: base appearance per ir/exemplars/PipelineStrip.html */ }
diff --git a/adapters/plain-css/stubs/wn-sidebar.css b/adapters/plain-css/stubs/wn-sidebar.css
new file mode 100644
index 0000000..ea51343
--- /dev/null
+++ b/adapters/plain-css/stubs/wn-sidebar.css
@@ -0,0 +1,4 @@
+/* @generated STUB by adapters/plain-css (T10) from ir/components/Sidebar.json — write-once. */
+/* Sidebar: Sidebar — extracted from designbook ui_kits/whynot-control/Chrome.jsx. */
+.wn-sidebar { /* TODO: base appearance per ir/exemplars/Sidebar.html */ }
+.wn-sidebar--doc: { /* current=doc: */ }
diff --git a/adapters/plain-css/stubs/wn-stage-dot.css b/adapters/plain-css/stubs/wn-stage-dot.css
new file mode 100644
index 0000000..e481bac
--- /dev/null
+++ b/adapters/plain-css/stubs/wn-stage-dot.css
@@ -0,0 +1,3 @@
+/* @generated STUB by adapters/plain-css (T10) from ir/components/StageDot.json — write-once. */
+/* StageDot: StageDot — extracted from designbook ui_kits/whynot-control/Atoms.jsx. */
+.wn-stage-dot { /* TODO: base appearance per ir/exemplars/StageDot.html */ }
diff --git a/adapters/plain-css/stubs/wn-stamp.css b/adapters/plain-css/stubs/wn-stamp.css
new file mode 100644
index 0000000..2f5592c
--- /dev/null
+++ b/adapters/plain-css/stubs/wn-stamp.css
@@ -0,0 +1,3 @@
+/* @generated STUB by adapters/plain-css (T10) from ir/components/Stamp.json — write-once. */
+/* Stamp: Stamp — extracted from designbook ui_kits/whynot-control/Atoms.jsx. */
+.wn-stamp { /* TODO: base appearance per ir/exemplars/Stamp.html */ }
diff --git a/adapters/plain-css/stubs/wn-tag.css b/adapters/plain-css/stubs/wn-tag.css
new file mode 100644
index 0000000..958ad3a
--- /dev/null
+++ b/adapters/plain-css/stubs/wn-tag.css
@@ -0,0 +1,3 @@
+/* @generated STUB by adapters/plain-css (T10) from ir/components/Tag.json — write-once. */
+/* Tag: Tag — extracted from designbook ui_kits/whynot-control/Atoms.jsx. */
+.wn-tag { /* TODO: base appearance per ir/exemplars/Tag.html */ }
diff --git a/adapters/plain-css/stubs/wn-top-nav.css b/adapters/plain-css/stubs/wn-top-nav.css
new file mode 100644
index 0000000..cc2a9e4
--- /dev/null
+++ b/adapters/plain-css/stubs/wn-top-nav.css
@@ -0,0 +1,3 @@
+/* @generated STUB by adapters/plain-css (T10) from ir/components/TopNav.json — write-once. */
+/* TopNav: TopNav — extracted from designbook ui_kits/whynot-control/Chrome.jsx. */
+.wn-top-nav { /* TODO: base appearance per ir/exemplars/TopNav.html */ }
diff --git a/adapters/plain-css/tokens.css b/adapters/plain-css/tokens.css
new file mode 100644
index 0000000..9d08898
--- /dev/null
+++ b/adapters/plain-css/tokens.css
@@ -0,0 +1,91 @@
+/* @generated by adapters/plain-css (WHYNOT-WP-0002 T10) from ir/tokens.json — DO NOT EDIT. */
+:root {
+  /* color */
+  --ink: #0A0A0A;
+  --ink-2: #1F1F1F;
+  --ink-3: #5C5C5C;
+  --ink-4: #8A8A8A;
+  --ink-5: #B5B5B3;
+  --line: #E5E5E2;
+  --line-strong: #C9C9C5;
+  --line-soft: #F0F0EC;
+  --paper: #FFFFFF;
+  --paper-2: #FAFAF7;
+  --paper-3: #F4F4EF;
+  --fg-1: var(--ink);
+  --fg-2: var(--ink-3);
+  --fg-3: var(--ink-4);
+  --fg-mute: var(--ink-5);
+  --fg-on-dark: #FAFAF7;
+  --bg-1: var(--paper);
+  --bg-2: var(--paper-2);
+  --bg-3: var(--paper-3);
+  --bg-invert: var(--ink);
+  --border: var(--line);
+  --border-strong: var(--line-strong);
+  --border-soft: var(--line-soft);
+  --hi: #FFE14A;
+  --hi-2: #FFD400;
+  --hi-ink: #1A1500;
+  --status-raw: #B5B5B3;
+  --status-weak: #8A8A8A;
+  --status-medium: #5C5C5C;
+  --status-strong: #0A0A0A;
+  --status-commercial: #FFD400;
+  --status-error: #B33A2E;
+  --status-error-bg: #FCF3F1;
+  --status-warn: #C28000;
+  --status-warn-bg: #FFFCEB;
+  --status-success: #2F6B3A;
+  --status-success-bg: #F2F7F2;
+  --status-info: #2E5C8A;
+  --status-info-bg: #F2F5FA;
+  /* fontFamily */
+  --ff-sans: ui-sans-serif, system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
+  --ff-mono: ui-monospace, "SF Mono", Menlo, Consolas, monospace;
+  --ff-serif: ui-serif, Georgia, "Times New Roman", serif;
+  /* fontSize */
+  --fs-xs: 11px;
+  --fs-sm: 13px;
+  --fs-base: 15px;
+  --fs-md: 17px;
+  --fs-lg: 20px;
+  --fs-xl: 24px;
+  --fs-2xl: 32px;
+  --fs-3xl: 44px;
+  --fs-4xl: 64px;
+  --fs-5xl: 96px;
+  /* lineHeight */
+  --lh-tight: 1.05;
+  --lh-snug: 1.25;
+  --lh-base: 1.5;
+  --lh-loose: 1.7;
+  /* letterSpacing */
+  --tr-tight: -0.02em;
+  --tr-snug: -0.01em;
+  --tr-base: 0em;
+  --tr-mono: 0.02em;
+  --tr-label: 0.08em;
+  /* space */
+  --sp-1: 4px;
+  --sp-2: 8px;
+  --sp-3: 12px;
+  --sp-4: 16px;
+  --sp-5: 24px;
+  --sp-6: 32px;
+  --sp-7: 48px;
+  --sp-8: 64px;
+  --sp-9: 96px;
+  --sp-10: 128px;
+  /* radius */
+  --r-0: 0px;
+  --r-1: 2px;
+  --r-2: 4px;
+  --r-3: 8px;
+  --r-pill: 999px;
+  /* shadow */
+  --shadow-0: none;
+  --shadow-1: 0 1px 0 var(--line);
+  --shadow-2: 0 1px 0 var(--line-strong);
+  --shadow-3: 0 4px 12px -6px rgba(10,10,10,0.10);
+}