<!DOCTYPE html>
<meta charset=utf-8>
<meta name="viewport" content="width=device-width, minimum-scale=0.25">
<style>
html {
  overflow: hidden;
}
html, body {
  margin: 0;
  width: 100%;
  height: 100%;
}
div {
  position: absolute;
}
</style>
<div style="background: red; width: 300%; height: 300%;"></div>
<div style="background: green; width: 200%; height: 200%;"></div>
<div style="background: blue; width: 100%; height: 100%;"></div>
<script>
window.onload = () => {
  parent.postMessage({ width: window.visualViewport.width,
                       height: window.visualViewport.height } , "*");
};
</script>