HTML结构如下

　　js逻辑也非常简单，就是移动left和top的值，代码如下

!function($dom){	var winWidth = window.innerWidth;                    	var winHeight = window.innerHeight;	window.onresize = function(){		winWidth = window.innerWidth;                    		winHeight = window.innerHeight;	}	var $left = 0,$top = 0,a = 1,b = 1,speed = 2;	function goFun(posit,speed){		return posit + speed;	}	var t = setInterval(function(){		$left = goFun($left,speed*a)		if( $left >= (winWidth-$dom.offsetWidth)){			a = -1;		}else if($left <= 0){			a = 1;		}		$top = goFun($top,speed*b)		if( $top >= (winHeight-$dom.offsetHeight)){			b = -1;		}else if($top <= 0){			b = 1;		}		$dom.style.cssText += "left:"+$left+"px;top:"+$top+"px";	},20)}(document.getElementsByClassName('fafa')[0]);