<button>點擊我</button>
<input type="button" value="按鈕">

button {
width: 100px;
}
input[type="button"] {
width: 50%;
}

button {
width: auto;
}
input[type="button"] {
display: block;
}

.btn {
width: 80px;
}
<button class="btn">按鈕一</button>
<button class="btn">按鈕二</button>
<button class="btn">按鈕三</button>