为什么CSS3动画不起作用？

Question

有人可以帮我找出为什么<h5>元素上的动画不起作用？

#hero h5 {
  animation: fadein 2s;
  -moz-animation: fadein 2s; /* Firefox */
  -webkit-animation: fadein 2s; /* Safari and Chrome */
  -o-animation: fadein 2s; /* Opera */
  font-weight: strong;
  font-size: 28px; 
}

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
<section id="hero">
    <div class="content">
        <div class="container">
            <div class="row">
                <h5>Lorem Ipsum Demo Title</h5>
            </div><!-- row -->
        </div> <!-- container -->
    </div> <!-- content -->
</section><!-- section -->      

Answer 1

您fadein在代码中调用动画,但您没有在任何地方定义它.

CSS3动画是用@keyframes规则定义的.有关CSS3动画的更多信息,请点击此处.

添加以下css:

@keyframes fadein {
  from {
    opacity: 0;
  }
  to {
    opacity: 1;
  }
}

#hero h5 {
  animation: fadein 2s;
  -moz-animation: fadein 2s; /* Firefox */
  -webkit-animation: fadein 2s; /* Safari and Chrome */
  -o-animation: fadein 2s; /* Opera */
  font-weight: strong;
  font-size: 28px; 
}

@keyframes fadein {
  from {
    opacity: 0;
  }
  to {
    opacity: 1;
  }
}

<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" rel="stylesheet"/>
<section id="hero">
  <div class="content">
    <div class="container">
      <div class="row">
        <h5>Lorem Ipsum Demo Title</h5>
      </div><!-- row -->
    </div> <!-- container -->
  </div> <!-- content -->
</section><!-- section -->