width is not always what is passed in

1. Go to https://www.khanacademy.org/computer-programming/new/pjs?width=600
2. Type this code:

``` println(width);
println(height);
```

In Chrome, the width ends up being 598. In Firefox, it's 600.

Ideally, it'd be 600, but students should also ideally not be dependent on exact widths.

Reproduce the issue at the linked Khan Academy URL using the provided println(width) and println(height) code in Chrome and Firefox. Trace how the requested width parameter becomes the value exposed to the program, then verify that the resulting width is consistent with the requested 600px behavior across both browsers.

Written by the indexing model from the issue text.