Making this solution “IE-compatible”

An extra wrapper is all we need to make this technique work in Internet Explorer versions 5, 6, and 7. (IE 8—as with other browsers—does not need this.)

CSS

#containingBlock {
	width: 50%;
}
.videoWrapper {
	position: relative;
	padding-top: 25px;
	padding-bottom: 56.25%;
	height: 0;
}
* html .videoWrapper {
	margin-bottom: 45px;
	margin-bot\tom: 0;
}
.videoWrapper div,
.videoWrapper embed, 
.videoWrapper object {
  position:absolute;
  width:100%;
  height:100%;
  left:0;
  top:0;
}

Markup

<div id="containingBlock">
<div class="videoWrapper">
  <div>
    <object width="480" height="295">
      <param name="movie" value="http://www.youtube.com/v/mDRYnaajUcY&hl=en&fs=1"></param>
      <param name="allowFullScreen" value="true"></param>
      <param name="allowscriptaccess" value="always"></param>
      <embed src="http://www.youtube.com/v/mDRYnaajUcY&hl=en&fs=1" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="480" height="295"></embed>
    </object>
  </div>
</div>