Canvas is stretched when using CSS but normal with "width" / "height" properties

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.

To set the width and height you need, you may use

canvasObject.setAttribute('width', '475');

For <canvas> elements, the CSS rules for width and height set the actual size of the canvas element that will be drawn to the page. On the other hand, the HTML attributes of width and height set the size of the coordinate system or 'grid' that the canvas API will use.

For example, consider this (jsfiddle):

var ctx = document.getElementById('canvas1').getContext('2d');
ctx.fillStyle = "red";
ctx.fillRect(10, 10, 30, 30);

var ctx2 = document.getElementById('canvas2').getContext('2d');
ctx2.fillStyle = "red";
ctx2.fillRect(10, 10, 30, 30);

canvas {
  border: 1px solid black;
}

<canvas id="canvas1" style="width: 50px; height: 100px;" height="50" width="100"></canvas>
<canvas id="canvas2" style="width: 100px; height: 100px;" height="50" width="100"></canvas>

Both have had the same thing drawn on them relative to the internal coordinates of the canvas element. But in the second canvas, the red rectangle will be twice as wide because the canvas as a whole is being stretched across a bigger area by the CSS rules.

Note: If the CSS rules for width and/or height aren't specified then the browser will use the HTML attributes to size the element such that 1 unit of these values equals 1px on the page. If these attributes aren't specified then they will default to a width of 300 and a height of 150.