validation on canvas element incorrectly flags 'width' attribute as an error

I'm trying to use a <canvas> element (for a Tiny Turtle exercise) and set its width to something larger than the default of 300px. Naively I tried a css rule:

canvas {
  width: 100%;
  min-height: 400px;
}

But with <canvas>, a css-specified width actually scales the content. This makes the turtle head and line segments appear quite large and even pixellated and does not actually give any more room to draw.

MDN recommends instead that you set the width and height directly on the element. Trying this triggered a popcode validation error on width:

9
Don't use the "width" attribute. Instead, use the CSS width property

Impact: mostly a little annoying. You can't set the real width of the canvas, so there will be unused space at the right margin -- and quite noticeable with a bigger screen. Complicated turtle drawings may not fit well into 300x150 px (although for some reason the height attribute is allowed through validation, so the canvas can be made taller).

Also, the validator-recommended fix isn't a real fix, which is minor in and of itself but we'd like users to be able to rely on its recommendations.

Steps to reproduce:

1. Open this popcode link: https://popcode.org/?gist=1791db30a9fa1cbcc39e0ec181388634
2. In the html panel, set the <canvas> element's width attribute to, say, 400. Observe the validation error.
3. Observe the somewhat-pixellated turtle stamp render (due to the css width being set w/o html width).
4. Observe the rendered length of a forward(30) segment. Cut that forward down to 10 units to allow more drawing room. Observe that the stamp now overwhelms the line segment.

On a page without validation, eg jsbin, setting the width attr has the desired result of setting the canvas's coordinate system width (which can then optionally be scaled with css).

Tested on macos 10.11 with Safari 10.0.3 and Chrome 56.0.2924.87.

Reproduce the validation error using the linked Popcode gist, focusing on a canvas element with a width attribute and the validator message recommending CSS width. Trace the validator entry point that emits this message and compare its treatment of width with height. Done means a valid canvas width is accepted and the recommendation no longer contradicts canvas sizing behavior.