<html>
  <body>
    <canvas id="canvas"></canvas>
  </body>
</html>
* {
  box-sizing: border-box;
}

html,
body,
canvas {
  display: block;
  position: relative;
  width: 100%;
  height: 100%;
  margin: 0;
  padding: 0;
}

canvas {
  background-color: rgb(80, 80, 80);
}
const canvas = document.getElementById("canvas");
const ctx = canvas.getContext("2d");
let width = window.innerWidth;
let height = window.innerHeight;
let mx = 0;
let my = 0;

const draw = () => {
  const eye = {
    x: width / 2,
    y: height / 2,
    radius: 50
  };

  const pupil = {
    x: eye.x,
    y: eye.y,
    radius: eye.radius / 2
  };

  ctx.clearRect(0, 0, width, height);

  ctx.beginPath();
  ctx.arc(eye.x, eye.y, eye.radius, 0, Math.PI * 2, true);
  ctx.fillStyle = "rgb(255, 255, 255)";
  ctx.fill();

  ctx.beginPath();
  ctx.arc(pupil.x, pupil.y, pupil.radius, 0, Math.PI * 2, true);
  ctx.fillStyle = "rgb(0, 0, 0)";
  ctx.fill();
  window.requestAnimationFrame(draw);
};

const handleResize = () => {
  width = window.innerWidth;
  height = window.innerHeight;

  /* Set the size on canvas instead of CSS to avoid strecthing */
  canvas.width = width;
  canvas.height = height;
};

const handleMousemove = e => {
  mx = e.clientX;
  my = e.clientY;
};

window.addEventListener("resize", handleResize);
window.addEventListener("mousemove", handleMousemove);
handleResize();
draw();

External CSS

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External JavaScript

This Pen doesn't use any external JavaScript resources.