<form>
<label>用戶名:</label>
<input type="text" name="username">
<br>
<label>密碼:</label>
<input type="password" name="password">
<br>
<input type="submit" value="登錄">
</form>

input[name="password"] {
margin-left: 30px;
}

input[name="submit"] {
position: absolute;
top: 50px;
right: 50px;
}

var input = document.getElementsByName("username")[0];
input.style.marginLeft = "100px";