javascript - Why is the body height 4 pixels more than the canvas height?

Question

css：

html,body {
  margin: 0;
  padding: 0;
}
canvas {
  margin: 0;
  padding: 0;
}

html：

<canvas id="canvas"></canvas>

javascript：

dt = document.getElementById('canvas');
dt.width = window.innerWidth;
dt.height = window.innerHeight;

The result is like this:

Logically shouldn’t canvas be as high as body? Why does body have 4 extra pixels?
Please tell me, thank you!

Answer 1

Add

to body

body {
    font-size: 0
}

Please see: http://www.w3cplus.com/css/fi...