HTML Code

<!DOCTYPE html>

<html lang=en>

<head>

  <meta charset=UTF-8>

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

  <title>poster — dynamic JS</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}video{display:block;border-radius:8px;margin-bottom:.75rem;max-width:100%}button{padding:.5rem 1rem;font:inherit;cursor:pointer;border-radius:6px;border:none;background:#2563eb;color:#fff}#status{margin-top:.75rem;padding:.75rem 1rem;border-radius:8px;font-size:.875rem;max-width:360px;background:#f0fdf4;border:1px solid #bbf7d0;color:#166534}</style>

</head>

<body>

  <h1>video.poster</h1>



  <video id=myVideo controls width=320 poster=/images/apple-80.png>

    <source src=/video/count.mp4 type=video/mp4>

    Your browser does not support the video tag.

  </video>



  <button type=button id=swapBtn>Change poster</button>

  <p id=status aria-live=polite>Current poster: /images/apple-80.png</p>



  <script>var video=document.getElementById("myVideo"),status=document.getElementById("status");document.getElementById("swapBtn").addEventListener("click",(function(){video.poster="/images/apple-120.png",status.textContent='poster updated to: "'+video.poster+'"'}))</script>

</body>

</html>

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HTML poster Attribute — dynamic JavaScript