JSFiddle - React, Tailwind, and code Playground

HTML

<div class="corner"></div>

CSS

.corner{
	width: 0; 
	height: 0; 
	border-top: 50px solid rgb(227, 37, 37);
	border-bottom: 50px solid transparent; 
	border-left: 50px solid transparent; 
	border-right:50px solid rgb(227, 37, 37); 
    float:right; 
}

.corner:hover{
	border-color: #A00404 #A00404 transparent transparent;
    transition: border-color 1s;
}