<form class="form-asd" role="form">
<h2 class="form-signin-heading">login</h2><hr />
<label class="control-label" for="username">Username</label>
<input class="form-control" type="email" required="" placeholder="username"data-error="enter username"></input>
<label class="control-label" for="username">password</label>
<input class="form-control" type="password" required=" " placeholder="Password"></input>
<label class="checkbox"></label>
<button class="btn btn-lg btn-primary " type="submit">submit</button>
</form>
我们如何将弹出必填字段"请填写此字段"的默认消息更改为"请输入用户名"