<button>點(diǎn)擊我</button>

<button id="btn" class="btn">點(diǎn)擊我</button>

.btn {
background-color: #4CAF50;
color: white;
}

.btn:hover {
background-color: #3E8E41;
}

<button id="btn" class="btn" onclick="alert('Hello World!')">點(diǎn)擊我</button>