利用伪元素与边框制作三角形

.nav_ul li:nth-child(1)::after {

content: '';

position: absolute;

left: 50px;

top: 3px;

width: 0;

height: 0;

border-width: 0 0 10px 10px;

border-style: solid;

border-color: red gold salmon transparent;

}