Bootstrap 表单样式

<h2>默认</h2>
  <label for="user">用户名</label>
  <input type="text">
  <h2>.form-control</h2>
  <label for="user">用户名</label>
  <input type="text" class="form-control">
  <h2>.form-group .form-control 配合 id 使用，点击用户名则可以唤起输入</h2>
  <div class="form-group">
    <label for="user">用户名</label>
    <input type="text" class="form-control" id="user" placeholder="用户名">
  </div>
  <div class="form-group">
    <label for="pass">密码</label>
    <input type="text" class="form-control" id="pass" placeholder="密码">
  </div>
  <h2>.sr-only </h2>
  <div class="form-group">
    <label for="user" class="sr-only">用户名</label>
    <input type="text" class="form-control" id="user" placeholder="用户名">
  </div>
  <div class="form-group">
    <label for="pass" class="sr-only">密码</label>
    <input type="text" class="form-control" id="pass" placeholder="密码">
  </div>
</form>