JSFiddle - React, Tailwind, and code Playground

HTML

<div class="wrapper">
  <div class="block"></div>
</div>

CSS

.wrapper {
  padding: 20px;
}

.block {
  width: 200px;
  height: 200px;
  border-radius: 5px;
  position: relative;
  overflow:hidden;
}

.block:before {
  content: '';
  border-style: solid;
  border-width: 24px 24px 0 0;
  border-color: #ffe066 transparent transparent transparent;
  display: block;
  height: 0;
  left: 0;
  position: absolute;
  top: 0;
  width: 0;
}