HTML Code
<!DOCTYPE html>

<html lang=en>

<head>

  <meta charset=UTF-8>

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

  <title>track.default in JavaScript</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{width:100%;max-width:360px;border-radius:8px;background:#0f172a}.controls{margin-top:.75rem;display:flex;flex-wrap:wrap;gap:.5rem}button{font:inherit;padding:.35rem .75rem;border:1px solid #cbd5e1;border-radius:6px;background:#fff;cursor:pointer}button.is-active{background:#2563eb;color:#fff;border-color:#2563eb}p{font-size:.875rem;color:#64748b;max-width:520px;margin-top:1rem}code{background:#e2e8f0;padding:.1rem .35rem;border-radius:4px}</style>

</head>

<body>

  <h1>Set default track in JavaScript</h1>

  <video id=demoVideo controls width=360 height=202>

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

    <track id=trackEn kind=subtitles src=/video/count.vtt srclang=en label=English default>

    <track id=trackEs kind=subtitles src=/video/count_es.vtt srclang=es label=Español>

  </video>

  <div class=controls>

    <button type=button id=btnEn class=is-active>Default: English</button>

    <button type=button id=btnEs>Default: Español</button>

  </div>

  <p>Click a button to set <code>track.default = true</code> on one track and <code>false</code> on the other, then reload captions.</p>

  <script>var video=document.getElementById("demoVideo"),trackEn=document.getElementById("trackEn"),trackEs=document.getElementById("trackEs"),btnEn=document.getElementById("btnEn"),btnEs=document.getElementById("btnEs");function setDefaultTrack(t,e){trackEn.default=t===trackEn,trackEs.default=t===trackEs,btnEn.classList.toggle("is-active",e===btnEn),btnEs.classList.toggle("is-active",e===btnEs),Array.prototype.forEach.call(video.textTracks,(function(e){e.mode=e===t?"showing":"disabled"}))}btnEn.addEventListener("click",(function(){setDefaultTrack(trackEn,btnEn)})),btnEs.addEventListener("click",(function(){setDefaultTrack(trackEs,btnEs)}))</script>

</body>

</html>
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HTML default Attribute — Toggle with JavaScript
