How to flip images horizontally with HTML5
canvas = document.createElement('canvas');
canvasContext = canvas.getContext('2d');
canvasContext.translate(width, 0);
canvasContext.scale(-1, 1);
canvasContext.drawImage(image, 0, 0);
Here's a snippet from a sprite object being used for testing and it produces the results you seem to expect.
Here's another site with more details. http://andrew.hedges.name/widgets/dev/
You don't need HTML5, it can be done with CSS same as in IE:
-moz-transform: scale(-1, 1);
-webkit-transform: scale(-1, 1);
-o-transform: scale(-1, 1);
transform: scale(-1, 1);
filter: FlipH;