screen.html

<!doctype html>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1, user-scalable=0">
<style>
html, body, iframe {
    margin: 0px;
    width: 100vw;
    height: 100vh;
}
#fsButton {
  position: fixed;
  z-index: 10;
  top: 20px; left:20px;
}
</style>

<button id="fsButton">Fullscreen</button>
<iframe id="frame" src="./index.html"></iframe>

<script>
    const fsButton = document.getElementById('fsButton');
    const iframe = document.getElementById('frame');
    const requestFullscreen = iframe.requestFullscreen
    || iframe.webkitRequestFullscreen || iframe.msRequestFullscreen;
    if (requestFullscreen === undefined) {
        fsButton.style.display = 'none';
    }
    fsButton.onclick = ()=>{
        requestFullscreen.call(iframe, {navigationUI: "hide" });
    };
</script>