Create contained blurred rectangle with HTML 5 canvas

canvas-blurred-rect.js

function blurredRect( w, h, blur, color ) {
	let c = document.createElement( 'canvas' );
	let ctx = c.getContext( '2d' );
	c.width = w;
	c.height = h;
	ctx.fillStyle = color;
	ctx.shadowBlur = blur;
	ctx.shadowColor = color;
	ctx.shadowOffsetX = w;
	ctx.shadowOffsetY = h;
	ctx.fillRect( -w + blur, -h + blur, w - blur * 2, h - blur * 2 );
	return c;
}

let br = blurRect( 400, 400, 100, '#000' );