ul { /* 取消ul預設的內縮及樣式 */ 
        margin: 0;
        padding: 0;
    }

    ul.drop-down-menu {
        display: inline-block;
        }

    ul.drop-down-menu li {
        position: relative;
    }

    ul.drop-down-menu > li:last-child {
        border-right: none;
    }

    ul.drop-down-menu > li {
        float: left; /* 只有第一層是靠左對齊*/
    }  
    
     ul.drop-down-menu a {
        display: block;        
    }
    ul.drop-down-menu a:hover { /* 滑鼠滑入按鈕變色*/
    }
    ul.drop-down-menu li:hover > a { /* 滑鼠移入次選單上層按鈕保持變色*/
    }
 
 ul.drop-down-menu ul {
       position: absolute;
       z-index: 99;
       top: 100%;

    }

    ul.drop-down-menu ul li {
       font-size:15px;
    }

    ul.drop-down-menu ul li:last-child {
        border-bottom: none;
    }

    ul.drop-down-menu ul ul { /*第三層以後的選單出現位置與第二層不同*/
        z-index: 999;
        top: 10px;
        left: 90%;
    }
    ul.drop-down-menu ul { /*隱藏次選單*/
        display: none;
    } 

    ul.drop-down-menu li:hover > ul { /* 滑鼠滑入展開次選單*/
        display: block;
    }