Canvas is stretched when using CSS but normal with `width` & `height` attributes

I have 2 canvases, one uses HTML attributes width and height to size it, the other uses CSS:

<canvas id="compteur1" width="300" height="300" onmousedown="compteurClick(this.id);"></canvas>
<canvas id="compteur2" style="width: 300px; height: 300px;" onmousedown="compteurClick(this.id);"></canvas>

Compteur1 displays like it should, but not compteur2. The content is drawn using JavaScript on a 300x300 canvas.

Why is there a display difference?

It seems that the width and height attributes determine the width or height of the canvas’s coordinate system, whereas the CSS properties just determine the size of the box in which it will be shown.

This is explained in the HTML specification:

The canvas element has two attributes to control the size of the element’s bitmap: width and height. These attributes, when specified, must have values that are valid non-negative integers. The rules for parsing non-negative integers must be used to obtain their numeric values. If an attribute is missing, or if parsing its value returns an error, then the default value must be used instead. The width attribute defaults to 300, and the height attribute defaults to 150.