GL-Matrix

HTML

<script src="https://raw.github.com/toji/gl-matrix/master/dist/gl-matrix-min.js"></script>
<script type="text/javascript" src="http://glmatrix.googlecode.com/hg/glMatrix.js"></script>

<div id="result"></div>

JavaScript

var matA = [1, 0, 0,
            0, 1, 0,
            1, 2, 1];

var matB = [1, 0, 0,
            0, 1, 0,
            3, 4, 1];

var out = [];

mat3.multiply(out, matA, matB);

document.getElementById('result').innerHTML = out;