html的input的required

添加这个required会成生什么效果，如下

代码如下：

<form id="registerForm">
    <input type="text" id="username" placeholder="用户名" required />
    <input type="password" id="password" placeholder="密码" required />
    <input type="email" id="email" placeholder="邮箱" required />
    <button type="submit">注册</button>
</form>