网页内容高度不够时，让 footer 处于页面底部；高度超出时，位于内容最后的方法

$footer_height: 80rpx;
$bottom_space: 40rpx;

.main {
  height: 100%;
}
.content {
  min-height: 100%;
  box-sizing: border-box;
  padding-bottom: $footer_height + $bottom_space;
}
.footer {
  height: $footer_height;
  margin-top: - ($footer_height + $bottom_space);
}

<div class="main">
  <div class="content">div>
  <div class="footer">div>
<div>