JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">

  <div class="button">
    Div fills the full width
  </div>

  <button class="button">
    Button does not fill the full width
  </button>

</div>

CSS

.container {
    background-color: #ddd;
    padding: 10px;
    max-width: 500px;
    margin-left: auto;
    margin-right: auto;
}

.button {
    background-color: #bbb;
    margin: 10px;
    padding: 10px
}