How to draw polygons on an HTML5 canvas?

Create a path with moveTo and lineTo (live demo):

var ctx = canvas.getContext('2d');
ctx.fillStyle="#f00";
ctx.beginPath();
ctx.moveTo(0, 0);
ctx.lineTo(100,50);
ctx.lineTo(50, 100);
ctx.lineTo(0, 90);
ctx.closePath();
ctx.fill();