puppeteer/test/assets/frames/nested-frames.html

<style>
:root {
  scrollbar-width: none;
}

body {
    display: flex;
}

body iframe {
    flex-grow: 1;
    flex-shrink: 1;
}
</style>
<script>
async function attachFrame(frameId, url) {
    var frame = document.createElement('iframe');
    frame.src = url;
    frame.id = frameId;
    document.body.appendChild(frame);
    await new Promise(x => frame.onload = x);
    return 'kazakh';
}
</script>
<iframe src='./two-frames.html' name='2frames'></iframe>
<iframe src='./frame.html' name='aframe'></iframe>