对于箭头,我使用了之前和之后的伪元素.以下是一些代码：

body {
  background: #FFFFFF;
  color: #FFFFFF;
  margin: 0px;
  padding: 0px;
  height: 100%;
}
.clear {
  clear: both;
}
.page-wrap {
  width: 980px;
  margin: 0px auto;
  height: 100%;
}
#main-menu {
  background: white;
  height: 55px;
  width: 100%;
  padding: 0px;
  margin: 0px;
  border-bottom: 1px solid black;
}
ul {
  font-family: Arial,Verdana;
  font-size: 18px;
  margin: 0;
  padding: 0;
  list-style: none;
}
ul li {
  display: block;
  position: relative;
  float: left;
}
li ul {
  display: none;
}
ul li a {
  display: block;
  text-decoration: none;
  color: black;
  padding: 0 9px 0 9px;
  background: white;
  margin-left: 1px;
  white-space: nowrap;
  line-height: 55px;
  font: 18px;
  font-family: Arial,Helvetica,sans-serif;
  outline: none;
}
ul li a:hover {
  color: black;
}
#menu a:hover:after {
  content: "";
  position: absolute;
  top: 40px;
  left: 50%;
  margin-left: -15px;
  width: 0px;
  height 0px;
  xxmargin: 0px auto;
  border-left: 15px solid transparent;
  border-right: 15px solid transparent;
  border-bottom: 15px solid black;
}

<header id="main-menu">
  <div class="page-wrap">
    <ul id="menu">
      <li><a href="#">Recommended</a></li>
      <li><a href="#">Recent</a></li>
    </ul>
  </div>
</header>

Fiddle Link

由于边框颜色,箭头为黑色.如何只显示箭头的边框？