Add constant-width-to-height-ratio snippet

snippets/constant-width-to-height-ratio.md

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+### Constant width to height ratio
+
+Given an element of variable width, it will ensure its height remains proportionate in a responsive fashion
+(i.e., its width to height ratio remains constant).
+
+#### HTML
+
+```html
+<div class="constant-width-to-height-ratio"></div>
+```
+
+#### CSS
+
+```css
+.constant-width-to-height-ratio {
+  background: #333;
+  width: 50%;
+  padding-top: 50%;
+}
+```
+
+#### Demo
+
+Resize your browser window to see the proportion of the element remain the same.
+
+<div class="snippet-demo">
+  <div class="snippet-demo__constant-width-to-height-ratio"></div>
+</div>
+
+<style>
+.snippet-demo__constant-width-to-height-ratio {
+  background: #333;
+  width: 50%;
+  padding-top: 50%;
+}
+</style>
+
+#### Explanation
+
+`padding-top` and `padding-bottom` can be used as an alternative to `height` such that the percentage value
+causes an element's height to become a percentage of its width, i.e. `50%` means the height will be 50% of the element's width. This allows its proportion to remain constant.
+
+#### Browser support
+
+<span class="snippet__support-note">⚠️ `padding-top` pushes any content within the element to the bottom.</span>