Canvas
.HTML
<canvas id="canvas"></canvas>
.Javascript
var canvas=document.getElementById('canvas')
var context=canvas.getContext('2d')
//使用context进行绘制
第二节 设置状态，stroke 笔画，绘制线条
Canvas 是基于状态进行绘制的
Draw a line
context.moveTo(100,100)
context.lineTo(700,700)
context.stroke()
//第三节 画圆
Draw an arc
画弧
context.arc(
conterx,centery,radius,
startingAngle,endingAngle,
anticlockwise=false
)