注意两个部分中间使用了一个全局变量bgc来记录表格高亮之前的颜色, 当鼠标移开后再变为原来的颜色.

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<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        td{
            width: 100px;
            height: 40px;
        }
    </style>
</head>
<body>
    <table border="1" style="border-collapse: collapse;">
        <tr>
            <td></td>
            <td></td>
            <td></td>
            <td></td>
            <td></td>
        </tr>
        <tr>
            <td></td>
            <td></td>
            <td></td>
            <td></td>
            <td></td>
        </tr>
        <tr>
            <td></td>
            <td></td>
            <td></td>
            <td></td>
            <td></td>
        </tr>
        <tr>
            <td></td>
            <td></td>
            <td></td>
            <td></td>
            <td></td>
        </tr>
        <tr>
            <td></td>
            <td></td>
            <td></td>
            <td></td>
            <td></td>
        </tr>
        <tr>
            <td></td>
            <td></td>
            <td></td>
            <td></td>
            <td></td>
        </tr>
        <tr>
            <td></td>
            <td></td>
            <td></td>
            <td></td>
            <td></td>
        </tr>
        <tr>
            <td></td>
            <td></td>
            <td></td>
            <td></td>
            <td></td>
        </tr>
    </table>

    <script>
        var trs = document.getElementsByTagName("tr");

        for(var i=0;i<trs.length;i++){

            if(i%2 == 0){
                trs[i].style.backgroundColor="pink";
            }
            else {
                trs[i].style.backgroundColor="gray";
            }

            //更改高亮之前的颜色
            var bgc;

            //鼠标移上高亮显示
            trs[i].onmouseover = function(){
                bgc = this.style.backgroundColor;
                this.style.backgroundColor="skyblue";
            }

            trs[i].onmouseout = function(){
                this.style.backgroundColor=bgc;
            }
        }

        
    </script>
</body>
</html>