var c = document.getElementsByTagName('canvas')[0];
var ctx = c.getContext('2d');
ctx.lineWidth = 1;
ctx.strokeStyle = '#f00';
ctx.fillStyle = '#eff';
ctx.fillRect( 10.5, 10.5, 20, 20 );
ctx.strokeRect( 10.5, 10.5, 20, 20 );
ctx.fillRect( 40, 10.5, 20, 20 );
ctx.strokeRect( 40, 10.5, 20, 20 );
ctx.fillRect( 70, 10, 20, 20 );
ctx.strokeRect( 70, 10, 20, 20 );
ctx.strokeStyle = '#fff';
ctx.strokeRect( 10.5, 10.5, 20, 20 );
ctx.strokeRect( 40, 10.5, 20, 20 );
ctx.strokeRect( 70, 10, 20, 20 );
body { background:#eee; margin:1em; text-align:center }
canvas { background:#fff; border:1px solid #ccc; width:400px; height:160px }
<canvas width="100" height="40"></canvas>
<p>Showing that re-drawing the same antialiased lines does not obliterate old antialiased lines.</p>