JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
Test Container
</div>

CSS

.container {
  background-color: #eee;
  padding: 2em;
}
.container:after {
  content: "";
  width: 0;
  height: 0;
  border-style: solid;
  border-width: 38px 20px 0 20px;
  border-color: yellow transparent transparent transparent;
  margin-left: .5em;
  display: inline-block;
  filter: drop-shadow(0 0 2.5px red) drop-shadow(0 0 0 red) drop-shadow(0 0 0 red) drop-shadow(0 0 0 red) drop-shadow(0 0 0 red) drop-shadow(0 0 0 red) drop-shadow(0 0 0 red) drop-shadow(0 0 0 red) drop-shadow(0 0 0 red);
}