只有內(nèi)嵌css有效，外部css會(huì)出現(xiàn)拉伸的情況，
所以有兩種方案

var myCanvas = "<canvas id='myCanvas' width='" + screen.availWidth + "px' height='"+ screen.availHeight + "px'></vanvas>";
document.body.insertAdjacentHTML("beforeEnd", myCanvas);
或者這種做法
var myCanvas = document.createElement("canvas");
myCanvas.setAttribute("width", screen.availWidth);
myCanvas.setAttribute("height", screen.availHeight);
myCanvas.setAttribute("id", "myCanvas");
document.body.appendChild(myCanvas);
兩種方案的共性都是在把元素添加進(jìn)DOM前設(shè)置他的大小

css樣式設(shè)置width:100%;height:100%;然后用下面四種方法皆可：
1.
canvas.width = document.documentElement.clientWidth;<br>
canvas.height = document.documentElement.clientHeight;
2.
canvas.width = screen.availWidth;<br>
canvas.height = screen.availHeight;
3.
canvas.width = screen.width;<br>
canvas.height = screen.height;
4.
canvas.width = window.innerWidth;<br>
canvas.height = window.innerHeight;