You can do it using a CSS3 3D transformation.
#videoElement
{
transform: rotateY(180deg);
-webkit-transform:rotateY(180deg); /* Safari and Chrome */
-moz-transform:rotateY(180deg); /* Firefox */
}
This will rotate it 180 degrees around its Y axis (so you're now looking at it from behind) which gives the same appearance as being mirrored.
Example at http://jsfiddle.net/DuT9U/1/
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