HTML:
<canvas id="canvas"></canvas>
JavaScript:
var canvas = document.getElementById('canvas'),
context = canvas.getContext('2d');
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
CSS:
* { margin:0; padding:0; } /* to remove the top and left whitespace */
html, body { width:100%; height:100%; } /* just to be sure these are full screen*/
canvas { display:block; } /* To remove the scrollbars */