A modal window can be implemented using a half-transparent `<div id="cover-div">` that covers the whole window, like this:

```css
#cover-div {
  position: fixed;
  top: 0;
  left: 0;
  z-index: 9000;
  width: 100%;
  height: 100%;
  background-color: gray;
  opacity: 0.3;
}
```

Because the `<div>` covers everything, it gets all clicks, not the page below it.

Also we can prevent page scroll by setting `body.style.overflowY='hidden'`.

The form should be not in the `<div>`, but next to it, because we don't want it to have `opacity`.
