em values

Sample:

Code:

<style>
 button {
  --btn-font-size: 1.2rem;

  border: 1px solid;
  border-radius: 4px;
  background: #123456;
  color: #ffffff;
  font-family: inherit;
  font-size: var(--btn-font-size);
  padding: 0.4em 0.8em;
}

 .btn--small {
  --btn-font-size: 1rem;
}

 .btn--large {
  --btn-font-size: 1.6rem;
}
</style>

<button class="btn--small">Save</button>
<button>Save</button>
<button class="btn--large">Save</button>