<style> 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'></iframe> <iframe src='./frame.html'></iframe>