CSS导航栏子栏

效果

HTML部分代码

            <li><a href="">产品中心</a><div class="mask">
                <p><a href="">流体设备</a></p>
                <p><a href="">微孔滤膜</a></p>
                <p><a href="">滤芯</a></p>
                <p><a href="">滤袋</a></p>
                <div id="delta"></div>
            </div></li>

css部分代码

#nav {
  width: 100%;
  height: 40px;
   /* background: #000; */
}
#nav > ul {
  width: 1200px;
  height: 100%;
  margin: 0 auto;
  /* background: lightsalmon */
}
#nav li {
  float: left;
  width: 130px;
  height: 100%;
  margin-right: 1px;
  text-align: center;
  font-size: 16px;
}
#nav li:nth-child(3) .mask {
width: 130px;
height: 172px;
background: #ababab;
margin-top: 10px; 
display: none;
z-index: 2;
}
#nav li:nth-child(3):hover .mask #delta{
  padding: 0;
  margin: 0;
  display: inline-block;
  width: 0;
  height: 0;
  border: 7px solid;
  border-color: transparent transparent #ababab transparent;
  left: 0;
  position: relative;
  top: -187px;
}
#nav li:nth-child(4):hover .mask #delta{
  padding: 0;
  margin: 0;
  display: inline-block;
  width: 0;
  height: 0;
  border: 7px solid;
  border-color: transparent transparent #ababab transparent;
  left: 0;
  position: relative;
  top: -273px;
}
#nav li:nth-child(3):hover .mask {
display: block;
}
#nav li:nth-child(3) .mask  p{
  line-height: 43px;
  height: 43px;
text-decoration: none;
/* background: #4b4b4b; */
}
#nav li:nth-child(3) .mask  p:hover{
background: #4b4b4b; 
}
#nav li:nth-child(3) .mask  p:nth-child(1):hover~#delta{
  border-color: transparent transparent #4b4b4b transparent;
}
#nav li:nth-child(3) .mask  a{
color: #fff;
text-decoration: none;
}