HTML5 - Canvas

onload = draw;

function draw() {
var canvas = document.getElementById('canvas');
if (!canvas || ! canvas.getContext) {
return false;
}
var ctx = canvas.getContext('2d');
ctx.beginPath();
ctx.moveTo(100, 100);
ctx.lineTo(300, 100);
ctx.lineTo(300, 300);
ctx.lineTo(100, 300);
ctx.lineTo(100, 100);
ctx.lineTo(300, 300);
ctx.moveTo(100, 300);
ctx.lineTo(300, 100);
ctx.stroke();
}