A mixin that maintains aspect ratio and always stays centered. You simply feed it a width and an aspect ratio.

Center Aspect

@mixin center_aspect($width, $aspect) {
	width: $width;
	$height: $width * $aspect;
	height: $height;
	max-height: 90%;
	left: 50%;
	top: 50%;
	margin-left: - ($width / 2);
	margin-top: - ($height / 2);
}