Styling siblings based on count and index

Historically, applying unique styles to each sibling in a list required complex :nth-child loops or JavaScript to inject inline styles. Modern CSS provides sibling-index() and sibling-count() to perform these calculations directly in your stylesheet, enabling dynamic layouts and color systems that automatically adapt as elements are added or removed.

Dynamic color systems

You can create a color spectrum across a group of siblings by calculating a unique hue or lightness value for each child. This ensures a consistent gradient effect regardless of the number of items.

.swatch {  /* Calculate hue by dividing the full 360deg circle by total siblings */  /* and multiplying by the current element's 1-based index */  background-color: hsl(    calc(360deg / sibling-count() * sibling-index()),    70%,    50%  );}

Symmetrical layout and fan effects

To create symmetrical effects (like a "fan" or centering items), use the total count to find the midpoint of the list.

.card {  /* Find the center index (e.g., 3 if there are 5 siblings) */  --center: calc((sibling-count() + 1) / 2);
  /* Rotate items away from the center: negative for left, positive for right */  /* center element gets 0deg rotation */  transform: rotate(calc(10deg * (sibling-index() - var(--center))));}

Circular and complex positioning

By combining these functions with CSS trigonometry (sin(), cos()), you can place elements in a perfect circle without any manual coordinates.

.orb {  /* Calculate the angle for this item's position on a 360deg circle */  --angle: calc(360deg / sibling-count() * sibling-index());  --radius: 150px;
  /* Set the pre-transformed position for all items to be centered */  position: absolute;  place-self: center;

  /* Position each element around the parent center */  transform: translate(    calc(cos(var(--angle)) * var(--radius)),    calc(sin(var(--angle)) * var(--radius))  );}

Fallback strategies

Baseline status for sibling-count() and sibling-index(): Newly available. It's been Baseline since 2026-08-18. Supported by: Chrome 138 (Jun 2025), Edge 138 (Jun 2025), Firefox 154 (Aug 2026), and Safari 26.2 (Dec 2025).

If sibling-index() and sibling-count() are not supported, provide a fallback by injecting CSS custom properties via JavaScript. MANDATORY: Use feature detection with CSS.supports() to ensure the script only runs when necessary.

/* MANDATORY: Check for native support before applying fallback */if (!CSS.supports('top: calc(sibling-index() * 1px)')) {  const items = document.querySelectorAll('.item');  items.forEach((item, index) => {    /* MANDATORY: Injected index must be 1-based to match native function */    item.style.setProperty('--sibling-index', index + 1);    item.style.setProperty('--sibling-count', items.length);  });}

In your CSS, use these variables as a base and override them with native functions inside an @supports block. MANDATORY: You MUST wrap the native function overrides in @supports to ensure the variables remain valid in older browsers.

.item {  /* 1. Set base values using variables (from JS fallback) */  --index: var(--sibling-index);  --count: var(--sibling-count);
  /* 2. Use the computed variables - replace this with your implementation-specific styles */  background-color: hsl(calc(360deg / var(--count) * var(--index)), 70%, 50%);}
@supports (top: calc(sibling-index() * 1px)) {  .item {    /* 3. Override with native functions ONLY if supported */    --index: sibling-index();    --count: sibling-count();  }}