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CSS matrix3d calculation is not right, but why?

I have the following JSFiddle http://jsfiddle.net/3vf9J/ which highlights my issue.

I've followed a guide on how to make a function to combine css transforms into a matrix3d so I can apple more than one at a time.

Unfortunately I've failing to make it work properly. A simple rotation of 180 degrees about one axis, ends up looking more like 135 degrees, so I've clearly got some maths wrong somewhere.

Can anyone who understands matrices help me out?

My function looks like this:

var generateRotationMatrix = function(x, y, z, tx, ty, tz) {
    var a = x;
    var b = y;
    var c = z;

    var rotationXMatrix = $M([
      [1,0,0,0],
      [0,Math.cos(a), Math.sin(-a), 0],
      [0,Math.sin(a), Math.cos( a), 0],
      [0,0,0,1]
    ]);

    var rotationYMatrix = $M([
      [Math.cos( b), 0, Math.sin(b),0],
      [0,1,0,0],
      [Math.sin(-b), 0, Math.cos(b), 0],
      [0,0,0,1]
    ]);

    var rotationZMatrix = $M([
      [Math.cos(c), Math.sin(-c), 0, 0],
      [Math.sin(c), Math.cos( c), 0, 0],
      [0,0,1,0],
      [0,0,0,1]
    ]);

    var translationMatrix = $M([
                [1,0,0,0],
                [0,1,0,0],
                [0,0,1,0],
                [tx,ty,tz,1]
                   ]);
    var tM = rotationXMatrix
             .x(rotationYMatrix)
             .x(rotationZMatrix)
         .x(translationMatrix);

    var s  = "matrix3d("
    s += tM.e(1,1).toFixed(10) + "," + tM.e(1,2).toFixed(10) + "," + tM.e(1,3).toFixed(10) + "," + tM.e(1,4).toFixed(10) + ","
    s += tM.e(2,1).toFixed(10) + "," + tM.e(2,2).toFixed(10) + "," + tM.e(2,3).toFixed(10) + "," + tM.e(2,4).toFixed(10) + ","
    s += tM.e(3,1).toFixed(10) + "," + tM.e(3,2).toFixed(10) + "," + tM.e(3,3).toFixed(10) + "," + tM.e(3,4).toFixed(10) + ","
    s += tM.e(4,1).toFixed(10) + "," + tM.e(4,2).toFixed(10) + "," + tM.e(4,3).toFixed(10) + "," + tM.e(4,4).toFixed(10)
    s += ")";
    return s;
}

Please note I'm using Sylvester to do my matrix maths (the multiplication)