Fantastic that we've finally got to the bottom of this one, also my test code no longer asserts when the code is changed to match your changes. So the rendering code becomes:

html  render: stylesheetFile contents

 
This test does not pass in my image either. I don't see where such
code would be used though? File (and thus CSS files) are certainly not
fed through a HTML canvas.

The test was my attempt at condensing the pertinent code relating to the problem. I'm fairly sure the code has a good correspondence with the code path used when you browse to:  http://localhost:8080/pier/environment/style.css, but as ever I might have missed something