<!DOCTYPE html>
<html>
<body>
<p>Test passes if you see a single 100px by 100px green box below.</p> 
<style>

#test {
    width: 100px;
    height: 100px;
    background: red;
}

div[slot=green] {
    width: 100px;
    height: 100px;
    background: green;
    color: green;
}

</style>
<div id="test"><div slot="green">PASS</div></div>
<script>
var rootElement = document.getElementById('test'); 
var root = rootElement.attachShadow({mode:'open'});
root.innerHTML = `
<style>
    #container { display: none; }
    #container.visible { display: block; }
</style>
<div><div id="container"><slot name="green">FAIL</slot></div></div>
`;

rootElement.getBoundingClientRect();
root.getElementById('container').classList.toggle('visible');

</script>
</body>
</html>