效果图：
<a href="#">
<span>HTML</span>
</a>
a {
position: relative;
text-decoration: none;
display: block;
text-align: center;
width: 200px;
}
/* 绘制上下线条 */
a::before,
a::after{
content: "";
position: absolute;
width: 100%;
height: 2px;
transition: width .2s ease-in-out;
background: #92B901;
}
/* 固定线条方向 */
a::before{
top:;
left:;
}
a::after{
bottom:;
right:;
}
/* 过滤效果 */
a:hover::before,
a:hover::after{
width:;
}
a span{
display: block;
padding: 20px;
}
/* 绘制左右线条 */
a span::before,
a span::after{
content: "";
position: absolute;
width: 2px;
height: 100%;
transition: height .2s ease;
background: #92B901;
}
a span::before{
bottom:;
left:;
}
a span::after{
top:;
right:;
}
a span:hover::before,
a span:hover::after{
height:;
}