I'm pulling what little is left of my hair out over this one.

Here's an example page.

Here's the source of the page in a gist.

The question, why doesn't the image scale to exactly fill the canvas?

Any help would be much appreciated.

Don Park got the answer

@davewiner it's because canvas element's dimensions are unspecified. add width="800" height="500" to the 'canvas' tag in HTML

— Don Park (@donpark) June 29, 2014

He's right, here's the corrected version of the page.