一些教程：
Firefox的教程：
https://developer.mozilla.org/zh-CN/docs/Web/API/Canvas_API
https://developer.mozilla.org/zh-CN/docs/Learn/JavaScript/Client-side_web_APIs/Drawing_graphics
菜鸟教程：
https://www.runoob.com/w3cnote/html5-canvas-intro.html
根据页面大小动态创建一个画布：
<canvas class="myCanvas">
<p>你的浏览器不支持canvas</p>
</canvas>
<script>
var canvas = document.querySelector('.myCanvas');
var width = canvas.width = window.innerWidth;
var height = canvas.height = window.innerHeight;
var ctx = canvas.getContext('2d');
</script>