Searching a little I have found this example from Mozilla Development Network
function draw() {
var ctx = document.getElementById('canvas').getContext('2d');
var radgrad = ctx.createRadialGradient(0,0,1,0,0,150);
radgrad.addColorStop(0, '#A7D30C');
radgrad.addColorStop(1, 'rgba(1,159,98,0)');
var radgrad2 = ctx.createRadialGradient(0,150,1,0,150,150);
radgrad2.addColorStop(0, '#FF5F98');
radgrad2.addColorStop(1, 'rgba(255,1,136,0)');
var radgrad3 = ctx.createRadialGradient(150,0,1,150,0,150);
radgrad3.addColorStop(0, '#00C9FF');
radgrad3.addColorStop(1, 'rgba(0,201,255,0)');
var radgrad4 = ctx.createRadialGradient(150,150,1,150,150,150);
radgrad4.addColorStop(0, '#F4F201');
radgrad4.addColorStop(1, 'rgba(228,199,0,0)');
ctx.fillStyle = radgrad4;
ctx.fillRect(0,0,150,150);
ctx.fillStyle = radgrad3;
ctx.fillRect(0,0,150,150);
ctx.fillStyle = radgrad2;
ctx.fillRect(0,0,150,150);
ctx.fillStyle = radgrad;
ctx.fillRect(0,0,150,150);
}
Based int this, you could draw each cell as a radial gradient and use a total transparent color as its final step so it blend better with other cells.
Without it, I think that you will need to calculate each pixel color based on how far from each cell they are.
Normally if when you make a voronoi texture, you divide the surface in a mesh and then assign a color to each vertex, then you interpolate the color of a pixel with the distance to the vertext that form its cell.
Also see http://www.raymondhill.net/voronoi/rhill-voronoi.html for an implementation of real voronoi in html5. It's open source and licensed under The MIT License, so you can use it.