<div class="form-group"> <h2>Please enter the password</h2> <p>Your browser must accept cookies in order to log in.</p> <input type="password" name="password" size="15"> <input type="submit" value="login"> </div> 整个页面上下左右都居中

.form-group{
  position:absolute;
  width:300px;
  height:200px;
  top:50%;
  left:50%;
  margin-top:-100px;
  margin-left:-150px;
}