The shadowBlur property

The shadowBlur property is used to set the severity of the blurring effect of the shadow

[No canvas support]
An example of the shadowBlur property

This sets the amount of blurring that the shadow has. In conjunction with the clip() function and setting the offset X/Y values to zero you can achieve inner shadows:

An example

    window.onload = function ()
        var canvas  = document.getElementById("cvs");
        var context = canvas.getContext('2d');

        var x       = 50;
        var y       = 50;
        var width   = 100;
        var height  = 100;
        context.shadowColor = '#ccc';
        context.shadowOffsetX = 3;
        context.shadowOffsetY = 3;
        context.shadowBlur = 25;
        context.fillStyle = 'red';    
        context.fillRect(x, y, width, height);