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+<!doctype html>
+<meta charset="utf-8">
+<title>09 framework input invariants</title>
+<link rel="stylesheet" href="../_style.css">
+<script src="../_lib.js"></script>
+<body>
+<main>
+  <p>Goal: type <code>abc</code>. Page asserts React-style invariants: <code>beforeinput</code>
+     fires <em>before</em> <code>input</code>, both per-character, <code>inputType==="insertText"</code>,
+     value mutates between events, and no <code>compositionstart</code> for plain typing.</p>
+  <input id="t" placeholder="type abc" style="font-size:18px;padding:8px 12px;width:240px">
+</main>
+<script>
+Challenge.init({ id: "composition-input", instructions: "type 'abc' producing framework-correct event order" });
+const t = document.getElementById("t");
+const evs = [];
+let sawCompositionStart = false;
+["beforeinput","input","compositionstart","compositionend"].forEach(name => {
+  t.addEventListener(name, (e) => {
+    evs.push({ name, t: e.timeStamp, inputType: e.inputType, data: e.data, value: t.value });
+    if (name === "compositionstart") sawCompositionStart = true;
+  });
+});
+t.addEventListener("input", () => {
+  if (t.value !== "abc") return;
+  const before = evs.filter(e => e.name === "beforeinput");
+  const inp = evs.filter(e => e.name === "input");
+  const bad = [];
+  if (sawCompositionStart) bad.push("compositionstart fired for plain ASCII typing");
+  if (before.length !== 3) bad.push(`beforeinput count=${before.length} (need 3)`);
+  if (inp.length !== 3) bad.push(`input count=${inp.length} (need 3)`);
+  if (before.some(b => b.inputType !== "insertText")) bad.push(`beforeinput.inputType=${before.map(b=>b.inputType).join(",")}`);
+  // beforeinput must precede matching input.
+  for (let i = 0; i < Math.min(before.length, inp.length); i++) {
+    if (before[i].t > inp[i].t) bad.push(`beforeinput[${i}] after input[${i}]`);
+  }
+  // Value must reflect each char incrementally.
+  const seq = inp.map(e => e.value).join("|");
+  if (seq !== "a|ab|abc") bad.push(`value seq at input events: ${seq} (need a|ab|abc)`);
+  Challenge.log("seq", { evs });
+  if (bad.length) Challenge.fail(...bad);
+  else Challenge.pass(`event order and invariants OK; seq=${seq}`);
+});
+</script>
+</body>