Add transform centering snippet

snippets/transform-centering.md

@@ -0,0 +1,67 @@
+### Transform Centering
+
+Vertically and horitonally centers a child element within its parent element using `position: absolute` and `transform: translate()` (as an alternative to `flexbox` or `display: table`). Similar to `flexbox`, this method does not require you to know the height or width of your parent or child so it is ideal for responsive applications.
+
+#### HTML
+
+```html
+<div class="parent">
+  <div class="child">Centered content</div>
+</div>
+```
+
+#### CSS
+
+```css
+.parent {
+  border: 1px solid #333;
+  height: 250px;
+  position: relative;
+  width: 250px;
+}
+
+.child {
+  left: 50%;
+  position: absolute;
+  top: 50%;
+  transform: translate(-50%, -50%);
+}
+```
+
+#### Demo
+<div class="snippet-demo">
+  <div class="snippet-demo__parent">
+    <div class="snippet-demo__child">Centered content</div>
+  </div>
+</div>
+
+<style>
+.snippet-demo__parent {
+  border: 1px solid #333;
+  height: 250px;
+  position: relative;
+  width: 250px;
+}
+.snippet-demo__child {
+  left: 50%;
+  position: absolute;
+  top: 50%;
+  transform: translate(-50%, -50%);
+}
+</style>
+
+#### Explanation
+
+1. `position: absolute` on the child element allows it to be positioned based on its containing block.
+2. `left: 50%` and `top: 50%` offsets the child 50% from the left and top edge of its containing block.
+3. `transform: translate(-50%, -50%)` allows the height and width of the child element to be negated so that it is vertically and horizontally centered.
+
+Note: Fixed height and width on parent element is for the demo only.
+
+#### Browser support
+
+<span class="snippet__support-note">⚠️ Requires prefix for full support.</span>
+
+* https://caniuse.com/#search=transform
+
+<!-- tags: layout -->