Add staggered animation snippet

snippets/staggered-animation.md

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+---
+title: Staggered animation
+tags: animation,advanced
+---
+
+Creates a staggered animation for the elements of a list.
+
+```html
+<div class="container">
+  <input type="checkbox" name="menu" id="menu" class="menu-toggler">
+  <label for="menu" class="menu-toggler-label">Menu</label>
+  <ul class="stagger-menu">
+    <li style="--i: 0">Home</li>
+    <li style="--i: 1">Pricing</li>
+    <li style="--i: 2">Account</li>
+    <li style="--i: 3">Support</li>
+    <li style="--i: 4">About</li>
+  </ul>
+</div>
+```
+
+```css
+.container {
+  overflow-x: hidden;
+  width: 100%;
+}
+
+.menu-toggler {
+  display: none;
+}
+
+.menu-toggler-label {
+  cursor: pointer;
+  font-size: 20px;
+  font-weight: bold;
+}
+
+.stagger-menu {
+  list-style-type: none;
+  margin: 16px 0;
+  padding: 0;
+}
+
+.stagger-menu li {
+  margin-bottom: 8px;
+  font-size: 18px;
+  opacity: 0;
+  transform: translateX(100%);
+  transition-property: opacity, transform;
+  transition-duration: 0.3s;
+  transition-timing-function: cubic-bezier(0.750, -0.015, 0.565, 1.055);
+}
+
+.menu-toggler:checked ~ .stagger-menu li {
+  opacity: 1;
+  transform: translateX(0);
+  transition-delay: calc(0.055s * var(--i));
+}
+```
+
+#### Explanation
+
+- Set the `opacity` to `0` and `transform` to `translateX(100%)` to make list elements transparent and move them all the way to the right.
+- Specify the appropriate `transition` properties for list elements, except `transition-delay` which is specified relative to the `--i` custom property.
+- Use inline styles to specify a value for `--i` for each list element, which will in turn be used for `transition-delay` to create the stagger effect.
+- Use the `:checked` selector for the checkbox to appropriately style list elements, setting `opacity` to `1` and `transform` to `translateX(0)` to make them appear and slide into view.
+
+#### Browser support
+
+- https://caniuse.com/#feat=css-variables
+- https://caniuse.com/#feat=css-transitions