JSFiddle - React, Tailwind, and code Playground

HTML

<form name="newsletter" target="#" method="post" class="group">
    <input name="email" type="text" placeholder="email here..." />
    <input name="submit" type="submit" class="button" value="subscribe" />
  </form>

CSS

* {
    box-sizing:border-box;
}

input[type="text"] {
    margin: 0;
    font-size: 0.75rem; /* 16x0.75=12px */
    width: 48%;
    padding: 0.625em;
    border: 1px solid black;
    border-radius: 0.3125em 0.3125em 0.3125em 0.3125em;
}

input[type="submit"] {
    margin: 0;
    width: 48%;
    font-size: 0.75rem; /* 16x0.75=12px */
    padding: 0.625em;
    border: none;
    border-radius: 0.3125em 0.3125em 0.3125em 0.3125em;
    text-transform: uppercase;
}