JSFiddle - React, Tailwind, and code Playground

HTML

<div class="parent"><div class="div"></div></div>

CSS

.div{
    width: 0;
height: 0;
border-style: solid;
border-width: 85px 85px 0 0;
border-color: #c00000 transparent transparent transparent;


top: 8px;
    border-radius: 0px 0px 0px 0px;
}
.parent{
 border-radius:10px 0px 0px 0px;  
   overflow:hidden; 
}