JSFiddle - React, Tailwind, and code Playground

HTML

<button class="button pop-up__button">btn</button>
<button class="pop-up__button button">btn</button>

CSS

* {
  box-sizing: border-box
}

html,
body {
  height: 100%;
  padding: 0;
  margin: 0;
}

.button {
  background: #f00;
}
.pop-up__button {
  background: #0f0;
}