diff --git a/simulator/static/flash.js b/simulator/static/flash.js
new file mode 100644
index 0000000..38ac605
--- /dev/null
+++ b/simulator/static/flash.js
@@ -0,0 +1,20 @@
+// Pure photosensitivity helper. WCAG 2.3.1: no more than 3 general flashes
+// (luminance transitions) per second. Given a transition COUNT, returns the
+// minimum total duration that keeps the rate at or below 3/sec, and a clamp
+// that only ever slows a transition down. UMD: browser `HEFFlash` + require().
+(function (root, factory) {
+  const api = factory();
+  if (typeof module !== "undefined" && module.exports) module.exports = api;
+  else root.HEFFlash = api;
+})(typeof self !== "undefined" ? self : this, function () {
+  "use strict";
+  const MAX_PER_SEC = 3;
+  function minSafeDurationMs(count) {
+    const n = Math.max(0, Number(count) || 0);
+    return (n / MAX_PER_SEC) * 1000;
+  }
+  function clampDurationMs(requestedMs, count) {
+    return Math.max(Number(requestedMs) || 0, minSafeDurationMs(count));
+  }
+  return { MAX_PER_SEC, minSafeDurationMs, clampDurationMs };
+});
diff --git a/simulator/static/flash.test.js b/simulator/static/flash.test.js
new file mode 100644
index 0000000..72f1188
--- /dev/null
+++ b/simulator/static/flash.test.js
@@ -0,0 +1,16 @@
+const test = require("node:test");
+const assert = require("node:assert");
+const F = require("./flash.js");
+
+test("minSafeDurationMs: N transitions need N/3 seconds", () => {
+  assert.strictEqual(F.minSafeDurationMs(3), 1000); // 3 flashes in >=1s
+  assert.strictEqual(F.minSafeDurationMs(6), 2000);
+  assert.strictEqual(F.minSafeDurationMs(0), 0);
+  assert.strictEqual(F.minSafeDurationMs(1), 1000 / 3);
+});
+
+test("clampDurationMs: stretches only when too fast", () => {
+  assert.strictEqual(F.clampDurationMs(2000, 3), 2000); // already safe
+  assert.strictEqual(F.clampDurationMs(200, 3), 1000);  // too fast -> clamped up
+  assert.strictEqual(F.clampDurationMs(500, 1), 500);   // single transition, slow enough
+});