Resize detection only works in Chrome now

I understand that at this point, this project seems to be no longer maintained. But I just want to point out, the code seems only work in Chrome (8/2019), which makes it a bit pointless because Chrome natively supports ResizeObserver. Firefox 68 and Edge (the non-chromium one) currently doesn't support ResizeObserver and this workaround also doesn't work with the two browsers. Here is my code in case I used in incorrectly:

```html

Why it doesn't work

.dummy {
width: 100%;
background: red;
position: relative;
}


const CSS = `position:absolute;left:0;top:-100%;width:100%;height:100%;margin:1px 0 0;border:none;opacity:0;pointer-events:none;`;

function observe(element, handler) {
const frame = document.createElement('iframe');
const supportsPE =
document.documentMode < 11 && 'pointerEvents' in frame.style;

frame.style.cssText = `${CSS}${supportsPE ? '' : 'visibility:hidden;'}`;
frame.onload = () => {
frame.contentWindow.onresize = () => {
handler(element);
};
};
element.appendChild(frame);
return frame;
};

observe(document.querySelector('.dummy'), function () {
console.log('dummy: resized!');
});

```

Start by running the supplied HTML reproduction in Firefox 68 and non-Chromium Edge, then compare its behavior with Chrome. Trace the detector entry point responsible for the iframe resize callback and establish whether the issue is reproducible; done means resize notifications work in the reported browsers or their unsupported status is clearly documented.

Written by the indexing model from the issue text.