LayoutTests/http/tests/images/image-supports-video.html

<picture>
    <source srcset="/resources/redirect-to-video-if-accepted.py?video=test.mp4" type="video/mp4">
    <source srcset="/resources/redirect-to-video-if-accepted.py?video=test.ogv" type="video/ogv">
    <img src="/resources/square100.png">
</picture>
<br>
<script>
if (window.testRunner) {
    testRunner.dumpAsText(true);
    testRunner.waitUntilDone();
}

function endTest() {
    if (window.testRunner)
        testRunner.notifyDone();
}

function logResult(text) {
    document.body.appendChild(document.createTextNode(text));
}

var img = document.querySelector('img');
img.addEventListener('load', event => {
    if (img.currentSrc.endsWith('square100.png'))
        logResult('FAIL: <img> fallback selected.');
    else
        logResult('PASS: video source selected.');

    endTest();
});

img.addEventListener('error', event => {
    logResult('FAIL: <img> threw error.');
    endTest();
});
</script>