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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>declare JavaScript Example</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}button{font:inherit;padding:.45rem .85rem;margin-right:.5rem;margin-bottom:.75rem;border:1px solid #cbd5e1;border-radius:6px;background:#2563eb;color:#fff;cursor:pointer}.output{max-width:480px;padding:.85rem 1rem;background:#fff;border:1px solid #e2e8f0;border-radius:8px;font-size:.875rem}p{font-size:.875rem;color:#64748b;max-width:480px;margin-top:1rem}</style>

</head>

<body>

  <h1>Toggle declare attribute</h1>

  <object id=dynamicObject data=/video/count.mp4 type=video/mp4 width=320 height=180>

    <p>Fallback content</p>

  </object>

  <div style=margin-top:.75rem>

    <button type=button id=addBtn>Add declare</button>

    <button type=button id=removeBtn>Remove declare</button>

  </div>

  <div class=output id=status>declare present: no</div>



  <script>var obj=document.getElementById("dynamicObject"),status=document.getElementById("status");function updateStatus(){status.textContent="declare present: "+(obj.hasAttribute("declare")?"yes":"no")}document.getElementById("addBtn").addEventListener("click",(function(){obj.setAttribute("declare",""),updateStatus()})),document.getElementById("removeBtn").addEventListener("click",(function(){obj.removeAttribute("declare"),updateStatus()}))</script>



  <p>Modern browsers do not change object loading behavior based on <code>declare</code>.</p>

</body>

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HTML declare Attribute — Toggle with JavaScript