<style?type="text/css"> ????#div1{ ????????width:?400px; ????????height:?300px; ????????border:?1px?solid?skyblue; ????} ????p{ ????????border:?1px?solid?lightpink; ????} ????.ps1{ ????????border:?1px?solid?red; ????????text-align:?center; ????} </style><input?type="button"?value="按鈕"?id="btn"?/> <div?id="div1"> ????<p?class="ps">我是一個段落</p> ????<p?class="ps">我是一個段落</p> ????<p?class="ps">我是一個段落</p> ????<p?class="ps">我是一個段落</p> ????<p?class="ps">我是一個段落</p> ????<p?class="ps">我是一個段落</p> </div><script?type="text/javascript"> ????//獲取按鈕 ????document.getElementById("btn").onclick=function?()?{ ????????//根據(jù)className獲取P ????????var?pObj=document.getElementsByClassName("ps"); ????????for?(var?i=0;i<pObj.length;i++){ ????????????pObj[i].className="ps1"; ????????} ????} </script>為什么最后出來的效果是這樣的？不是每個p標簽都應用了ps1類樣式，而是跳過一個p應用一個。