JSFiddle - React, Tailwind, and code Playground

HTML

<div class="triangle"></div>
<div class="triangle_shadow"></div>

CSS

div.triangle {
    border-bottom : 60px solid transparent;
    border-left : 60px solid black;
    z-index: 1;
}

div.triangle_shadow {
    border-bottom : 60px solid transparent;
    border-left : 60px solid red;
    margin-top: -50px;
   margin-left:10px;
   filter: blur(5px);
   z-index: -1;
}