Centered (h&v) 16:9 iframe that uses as much screen estate as possible but refrains from being cropped anywhere

Requirements:

HTML:

<div class="container">
    <iframe></iframe>
</div>

CSS:

.container {
   width:90vw;
   height:50vh;
}

Now if I resize the browser (vertically OR horizontally) the video should not be cropped and no scrollbars should appear.

How to achieve this?