The following CSS will work in IE and modern browsers that support CSS transforms. I included a vertical flip class just in case you might want to use it too.
.flip-horizontal {
-moz-transform: scaleX(-1);
-webkit-transform: scaleX(-1);
-o-transform: scaleX(-1);
transform: scaleX(-1);
-ms-filter: fliph; /*IE*/
filter: fliph;
}
.flip-vertical {
-moz-transform: scaleY(-1);
-webkit-transform: scaleY(-1);
-o-transform: scaleY(-1);
transform: scaleY(-1);
-ms-filter: flipv; /*IE*/
filter: flipv;
}