42-modify-fill-mode-of-an-animation.html

<style>
  button {
    border-radius: 5px;
    color: white;
    background-color: #0F5897;
    padding: 5px 10px 8px 10px;
  }

  button:hover {
    animation-name: background-color;
    animation-duration: 500ms;

    animation-fill-mode: forwards;
  }

  @keyframes background-color {
    100% {
      background-color: #4791d0;
    }
  }
</style>

<button>Register</button>