JSFiddle - React, Tailwind, and code Playground

HTML

<div>29</div>

CSS

div {
   width:50px;
   height:50px; 
   border:1px solid black;
   position:relative;
   text-align:center;
   line-height:50px;
   font-size:25px;
   background:linear-gradient(to bottom right, white, white 48%, black 50%, white 52%, white);
}