JSFiddle - React, Tailwind, and code Playground

HTML

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

CSS

.block {
  border: 1px solid green;
  border-radius: 10px;
  padding: 20px;
  position: relative;
}

.block::before,
.block::after {
  content: '';
  position: absolute;
  left: 20px;
  bottom: -20px;
  border: 10px solid transparent;
  border-top: 10px solid green;
}

.block::after {
  border-top: 10px solid white;
  bottom: -19px;
}