.nav_1::after{
content: " ";
position: absolute;
left: 0;
bottom: 0;
width: 100%;
height: 0;
border-bottom: 1px solid #b4b4b4;
-webkit-transform: scaleY(0.5);
transform: scaleY(0.5);
}
以上前提必须给此元素加上样式position---->relative.