登录页面
<br>
<form method="post" action="./">
用户名:
<br>
<input type="text" name="username" id="lower" required>
<br>
密码:
<br>
<input type="password" name="passwd" required>
<br>
<button type="submit">登录</button>
</form>

<script>
    document.getElementById('lower').addEventListener('input', function() {
        this.value = this.value.toLowerCase();
    });
</script>