Canvas border should always be visible

If you use an image with a faint border, it's hard to tell exactly where the border of the image is because the canvas resizes to fit the image but lacks a high-contrast border. My own suggestion would be to use a little CSS:

``` css
canvas {
    border: 1px solid black;
}
```

To reproduce this issue, try a solid white image -- it vanishes completely against the page's default background.
