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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>marginwidth — legacy iframe</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}.warn{max-width:520px;padding:.75rem 1rem;background:#fffbeb;border:1px solid #fcd34d;border-radius:8px;font-size:.8125rem;color:#92400e;margin-bottom:1rem}iframe{border:2px solid #64748b;border-radius:6px;background:#fff}.hint{margin-top:1rem;font-size:.8125rem;color:#64748b;max-width:520px}code{background:#e2e8f0;padding:.1rem .35rem;border-radius:4px}</style>
</head>
<body>
  <h1>marginwidth on &lt;iframe&gt;</h1>
  <p class=warn><strong>Deprecated:</strong> <code>marginwidth</code> is obsolete in HTML5. Modern browsers may ignore it — use CSS padding instead.</p>
  <iframe srcdoc="&lt;!DOCTYPE html&gt;&lt;html&gt;&lt;body style='margin:0;font-family:system-ui;padding:8px;background:#dbeafe;'&gt;Embedded page content&lt;/body&gt;&lt;/html&gt;" width=320 height=120 marginwidth=24 title="Demo iframe">
  </iframe>
  <p class=hint><code>marginwidth="24"</code> was meant to add 24px left and right space <em>inside</em> the iframe, between the border and the embedded document.</p>
</body>
</html>

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HTML marginwidth Attribute — Legacy iframe Spacing