button {
  opacity: 0.8;
  background-color: #4CAF50; /* Green */
  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) => {
  e.preventDefault();
  alert('a');
})


document.body.addEventListener("mousedown", () => {
  document.body.classList.add("using-mouse");
});
document.body.addEventListener("keydown", ({ key }) => { 
  if (key === "Tab") { 
    document.body.classList.remove("using-mouse"); 
  } 
});