画出倒三角

css:

.header_menu_new-top{ 0rem;height: 0rem;position: absolute;top: -0.12rem;right: 0.2rem;border-color: rgba(0,0,0,0.8) transparent;border- 0rem 0.12rem 0.12rem 0.12rem;border-style:solid;}

html:

<div class="header_menu_new-top"></div>