The following iframe is a flex item and is supposed to stretch and fill the available space:

Code:
<!DOCTYPE html>
<html>

<head>
    <meta charset="UTF-8">
    <title>Flex Iframe</title>
    <style>
        body {
            display: flex;
            margin: 0;
            height: 100vh;
        }
        span {
            background: green;
        }
        iframe {
            background: tan;
        }
    </style>
</head>

<body>
    <span>Hello, world!</span>
    <iframe></iframe>
</body>

</html>
But in IE11 it doesn't look right:
DEMO

Is it a bug? What's a cross-browser solution?