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<!DOCTYPE html>

<html lang=en>

<head>

  <meta charset=UTF-8>

  <meta name=viewport content="width=device-width,initial-scale=1">

  <title>oninvalid — addEventListener</title>

  <style>body{font-family:system-ui,sans-serif;margin:0;padding:1.5rem;background:#f8fafc;color:#334155;line-height:1.7}h1{margin:0 0 .75rem;font-size:1.25rem;color:#0f172a}label{display:block;font-size:.8125rem;font-weight:500;margin-bottom:.35rem}input{display:block;width:100%;max-width:280px;padding:.5rem .75rem;border:1px solid #cbd5e1;border-radius:8px;font-size:.875rem;margin-bottom:.75rem}button{padding:.45rem .85rem;border:1px solid #2563eb;border-radius:8px;background:#2563eb;color:#fff;font-size:.8125rem;cursor:pointer}.status{margin-top:.75rem;padding:.75rem 1rem;background:#fef2f2;border:1px solid #fecaca;border-radius:8px;font-size:.8125rem;max-width:420px}</style>

</head>

<body>

  <h1>dynamicField + addEventListener</h1>

  <p>Click Validate with an empty required field.</p>

  <label for=dynamicField>Name</label>

  <input id=dynamicField required>



  <button type=button id=checkBtn>Validate</button>

  <div class=status id=status>Handler registered in script — not inline.</div>



  <script>var field=document.getElementById("dynamicField"),status=document.getElementById("status");field.addEventListener("invalid",(function(){field.setCustomValidity("This field is invalid. Please provide a valid input."),status.textContent="invalid event fired — custom message set."})),field.addEventListener("input",(function(){field.setCustomValidity(""),status.textContent="Input cleared — field can validate again."})),document.getElementById("checkBtn").addEventListener("click",(function(){field.reportValidity()?status.textContent="Field is valid!":status.textContent="Validation failed — see browser message."}))</script>

</body>

</html>

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HTML oninvalid Attribute — addEventListener