If you don't like the jaggies in your HTML Canvas pictures, you can make the canvas size be twice as big and use CSS to set its size on the screen back to what you want.

Then you get nicely antialiased graphics.

Don't go crazy on the expansion though or iPhone users will hate you.

You should also be careful to drop your endpoints of horizontal and vertical lines on the xxxx.5 boundaries (in screen space) to keep nice sharp lines, otherwise you get a two pixel wide smear.