html：

<h1>垂直翻转</h1>
<div class="card vertical">
  <div class="card-side front">
    <div>正面</div>
  </div>
  <div class="card-side back">
    <div>背面</div>
  </div>
</div>

<h1>水平翻转</h1>
<div class="card horizontal">
  <div class="card-side front">
    <div>正面</div>
  </div>
  <div class="card-side back">
    <div>背面</div>
  </div>
</div>

css:

/*卡片样式*/
.card {
  perspective: 150rem;
  position: relative;
  height: 20rem;
  max-width: 200px;
  box-shadow: none;
  background: none;
}

.card-side {
  height: 10rem;
  border-radius: 15px;
  transition: all 0.8s ease;
  backface-visibility: hidden;
  position: absolute;
  top: 0;
  left: 0;
  width: 80%;
  padding:2rem;
  color: white
}

/*正面背景*/
.card-side.front {
  background-color: #0093E9;
  background-image: linear-gradient(160deg, #0093E9 0%, #80D0C7 100%);
}
/*背面背景*/
.card-side.back {
  background-color: #4158D0;
  background-image: linear-gradient(43deg, #4158D0 0%,#C850C0 46%, #FFCC70 100%);
}

/*垂直翻转*/
.vertical .back{
  transform: rotateX(-180deg);
}
.vertical:hover .card-side.front {
  transform: rotateX(180deg);
}
.vertical:hover .card-side.back {
  transform: rotateX(0deg);
}
/*水平翻转*/
.horizontal .back{
  transform: rotateY(-180deg);
}
.horizontal:hover .card-side.front {
  transform: rotateY(180deg);
}
.horizontal:hover .card-side.back {
  transform: rotateY(0deg);
}

总结

翻转卡片效果使用了以下css的知识点：

1. 背景色—— background-color
2. 背景渐变 —— background-image: linear-gradient()
3. css 转换 —— transform
4. css 旋转 —— rotateY、rotateX
5. css 透视 —— perspective