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

<!DOCTYPE html>

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

    <span>Hello, world!</span>

But in IE11 it doesn't look right:

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