include a canvas in a webpage:

<canvas id="a_canvas" width="300" height="225">

obtain a reference to the canvas to use with javascript:

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

draw a rectangle:

a_context.fillRect(x, y, width, height);

Canvas context properties

style	fillStyle	set or return the color, gradient, or pattern used to fill the drawing
style	strokestyle	set or return the color, gradient, or pattern used to draw lines
color	shadowColor	set or return the color used for shadows
color	shadowBlur	set or return the level used for shadow blurring
color	shadowOffsetX	set or return the offset of the shadow from the object
color	shadowOffsetY

Canvas context Methods

createLinearGradient()
createPattern()
createRadialGradient()
addColorStop()

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Resources
WWW Schools - HTML Canvas Reference
MDN - HTMLCanvasElement
Dive Into HTML5 #4 Let's call it a drawing surface