mouseenter事件只会触发一次，触发对象是注册对象或者注册对象的子元素
mouseover事件可以触发多次，触发对象是注册对象或者注册对象的子元素
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<title>Document</title>
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<p>不论鼠标指针穿过被选元素或其子元素，都会触发 mouseover 事件。</p>
<p>只有在鼠标指针穿过被选元素时，才会触发 mouseenter 事件。</p>
<div class="over" style="background-color:lightgray;padding:20px;width:40%;float:left">
<h2 style="background-color:white;">被触发的 Mouseover 事件：<span></span></h2>
</div>
<div class="enter" style="background-color:lightgray;padding:20px;width:40%;float:right">
<h2 style="background-color:white;">被触发的 Mouseenter 事件：<span></span></h2>
</div>
<script>
var x = 0,y = 0;
var div1 = document.getElementsByClassName("over")[0];
div1.addEventListener("mouseover",function(){
var span = this.getElementsByTagName("span")[0]
span.innerText = (x +=1);
span.style.cssText = "border:2px red solid;";
},false);
var div2 = document.getElementsByClassName("enter")[0];
div2.addEventListener("mouseenter",function(){
var span = this.getElementsByTagName("span")[0]
span.innerText = (y +=1);
span.style.cssText = "border:2px red solid;";
},false);
</script>
</body>
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