I thought this would be the answer--adding the viewport initial scale, but it's not. It does load and play just fine, but the initial view is still 150(?)%.
Anybody? Bueller?
<!doctype html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<style type="text/css">
body {
margin: 0px;
}
</style>
</head>
<body>
<iframe width="709" height="399" src="https://www.youtube.com/embed/Q9niPtOaPsw?rel=0" frameborder="0" allowfullscreen></iframe>
</body>
</html>