Responsive Iframes

<style type="text/css">
    .resp-container {
        position: relative;
        overflow: hidden;
        padding-top: 56.25%;
    }

    .resp-iframe {
        position: absolute;
        top: 0;
        left: 0;
        width: 100%;
        height: 100%;
        border: 0;
    }
</style>
<div class="resp-container">
    <iframe class="resp-iframe" src="https://www.youtube.com/embed/dQw4w9WgXcQ"></iframe>
</div>

padding-top：為 iframe 的 (高 / 寬 * 100%)，youtube 預設是 16:9 即 56.25%