<canvas id="a_canvas" width="300" height="300"></canvas>
<script type="text/javascript">

var canvas = document.getElementById("a_canvas");
var ctx=canvas.getContext("2d");

var g = ctx.createRadialGradient(30,30,20,170,170,100);
g.addColorStop(0.3, "red");
g.addColorStop(0.7, "yellow");
g.addColorStop(1.0, "blue");

ctx.fillStyle= g;
ctx.fillRect(20,20,260,260);

</script>