添加sr-only没有隐藏掉label，什么原因？

<form action="" role="form" class="form-inline">

<div class="form-group">

<label for="username sr-only">用户名</label>

<input class="form-control" type="text" id="username" placeholder="test">

</div>

<div class="form-group">

<label for="password">密码</label>

<input type="password" class="form-control" placeholder="password">

</div>

<div class="checkbox">

<label>

<input type="checkbox">记住密码

</label>

</div>

<button type="submit" class="btn btn-default">登录</button>

</form>