CSS 如何实现回到顶部按钮？

Question

CSS 如何实现回到顶部按钮？

仓乃文 · Accepted Answer

回到顶部按钮通过 position: fixed 固定在页面右下角，配合 scroll-behavior: smooth 实现平滑滚动到顶部。

```css
.back-to-top {
  position: fixed;
  bottom: 30px;
  right: 30px;
  width: 50px;
  height: 50px;
  background: #6c5ce7;
  color: #fff;
  border-radius: 50%;
  display: flex;
  align-items: center;
  justify-content: center;
  text-decoration: none;
  font-size: 24px;
  box-shadow: 0 2px 10px rgba(108, 92, 231, 0.4);
  opacity: 0;
  visibility: hidden;
  transition: all 0.3s;
  z-index: 999;
}

.back-to-top.visible {
  opacity: 1;
  visibility: visible;
}

html {
  scroll-behavior: smooth;
}
```

纯 CSS 箭头：
```css
.back-to-top::before {
  content: ";
  width: 12px;
  height: 12px;
  border: solid #fff;
  border-width: 2px 2px 0 0;
  transform: rotate(-45deg);
  margin-top: 4px;
}
```

补充说明：
- scroll-behavior: smooth 让锚点跳转平滑滚动
- visible 类通过 JS 根据滚动距离动态添加
- 箭头用 border + rotate 绘制，无需图片
- 适合长页面、文章详情页等场景