Centering a canvas

Question

How do I markup a page with an HTML5 canvas such that the canvas

1. Takes up 80% of the width

2. Has a corresponding pixel h

Answer 1

in order to center the canvas within the window +"px" should be added to el.style.top and el.style.left.

el.style.top = (viewportHeight - canvasHeight) / 2 +"px";
el.style.left = (viewportWidth - canvasWidth) / 2 +"px";