<button>hello</button>
button {
  background-color: #4CAF50; /* Green */
  opacity: 0.8;
  border: none;
  color: white;
  padding: 15px 32px;
  text-align: center;
  text-decoration: none;
  display: inline-block;
  font-size: 16px;
  transition-duration: 0.4s;
}

button:hover {
  background-color: #4CAF50; /* Green */
  opacity: 1;
  color: white;
}

button:focus {
  border: 2px solid blue;
}

body.using-mouse :focus {
  animation: none;
  border: none;
  outline: none;
}
document.querySelector('button').addEventListener('click', (e) => {
  alert('a');
})

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