长宽比（Ratio）
https://getbootstrap.com/docs/5.2/helpers/ratio/https://www.bootstrap.cn/doc/read/157.html
<div class="ratio ratio-1x1">
<div>1x1</div>
</div>
<div class="ratio ratio-4x3">
<div>4x3</div>
</div>
<div class="ratio ratio-16x9">
<div>16x9</div>
</div>
<div class="ratio ratio-21x9">
<div>21x9</div>
</div>
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