https://torunskihokej.pl/js/iframe_auto_scale.js

1function iframeAutoScale() {
2
3    'use strict';
4
5    var
6      iframes = document.getElementsByTagName('iframe'),
7      index = 0;
8
9    if (iframes.length > 0) {
10      for (index = 0; index < iframes.length; index++) {
11        var
12          iframe = iframes[index],
13          parent = iframe.parentElement,
14          parentPadding = parseInt(window.getComputedStyle(parent, null).getPropertyValue('padding-left')) + parseInt(window.getComputedStyle(parent, null).getPropertyValue('padding-right')),
15          parentWidth = parent.clientWidth - parentPadding,
16          ratio = 0.75,	// default ratio (4:3)
17          width = iframe.clientWidth,
18          height = iframe.clientHeight;
19
20        // overwrite default ratio if width and height attributes are not set
21        if (width !== undefined && height !== undefined) {
22          ratio = height / width;
23        }
24
25        iframe.setAttribute('width', parentWidth);
26        iframe.setAttribute('height', parentWidth * ratio);
27      }
28    }
29  }
30
31  // onload
32  document.addEventListener('DOMContentLoaded', function () {
33    iframeAutoScale();
34  }, false);
35
36  // on window resize
37  window.onresize = function(event) {
38    iframeAutoScale();
39  };