<a href="">İlk denememiz</a>
a{
  display:block;
  background-color:#c4342f;
  padding:5px 10px;
  width:100px;
  color:#fff;
  -webkit-transition: background-color 2s ease-in-out 1s;
  -moz-transition: background-color 2s ease-in-out 1s;
  transition: background-color 2s ease-in-out 1s;
}
a.tetikle {
  background-color:blue;
}
document.querySelector('a').className = 'tetikle';

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